The present invention relates to a multiple, in particular double, glazed windowpane assembly with an electrically operated blind arranged internally in said windowpane assembly, said windowpane assembly comprising two parallel windowpanes arranged on either side of a top box in which an electric motor and an operating mechanism for a blind are located, said windowpane assembly further comprising a sealed lead-in arrangement, comprising a lead-in block through which electrical current for the operation of the electric motor can be supplied. It is known to arrange electrically operated blinds internally in double glazed windowpane assemblies. Such an arrangement is e.g. known from WO-A-02/053865. In this arrangement the mechanical and electrical parts of the driving mechanism for the blind is housed in a top box forming part of the spacer arrangement between the two windowpanes of the double glazed windowpane assembly. When the windowpanes have been adhered to the spacer arrangement and the gap between the windowpanes outside the spacer arrangement has been filled with an appropriate sealing material, the windowpane assembly presents a sealed unit with a sealed cavity between the two windowpanes. However, the electrical leads, which must extend to the exterior, for electrical supply of the electric motor, present a weak point in the sealing of the windowpane assembly. In WO-A-02/053865 it is attempted to improve this sealing by providing a lead-in in form of a moulded end plug for the housing, the end plug serving also as a cor¬ ner piece for the top box, or rather for the lid thereof and the spacer profiles along the sides of the window pane assembly. The end plug is made of a moulded plastic material, in which a male connector is integrally formed by moulding-in of metal connector pins. The pins thus form a relatively stable sealed connection through the plug. In the male connector is placed a corresponding connector with flexible leads to the outside, and the whole assembly of housing, plug, connector and leads is sealed by means of a suitable material such as butyl or silicone. Due to the use of the plug and connector, any strain on the leads will not break the sealing, as it would be the case if the leads were simply sealed by bu¬ tyl or silicone around them alone. It is the object of the present invention to pro¬ vide a sealed lead-in arrangement in a multiple glazed windowpane assembly with electrically operated blinds ar- ranged internally in the windowpane assembly, which over¬ comes the above problems. According to the invention this object is, in a multiple glazed windowpane assembly according to the opening paragraph, achieved by said top box comprising a generally U-shaped housing part and a lid part adapted to interlock with and close said housing part at the open end of the U-shape, and by said lid comprising an aperture adapted to com¬ municate with a socket end of the lead-in block abutting the inside of said lid in a sealing manner around said aperture. Arranging the lead-in block within the housing al¬ lows it to be clamped tightly between the lid and the housing part, in such a manner that the lid, when it is snapped onto the housing exerts a force against the lead- in block around the aperture. This force will be main¬ tained over the lifetime of the windowpane assembly, thereby securing the sealing over time. Moreover, this arrangement has the advantage that the lead-in may be located at any appropriate place along the length of the top box. According to a preferred embodiment, the lead-in block has an abutment surface adapted for abutting the inside of the lid. Having an abutment surface abutting the inside of the lid provides good sealing when the re¬ siliency of the lid presses it against the abutment sur¬ face. According to a further preferred embodiment, the abutment surface lead-in block has a raised portion adapted to fit through the aperture in the lid. This al¬ lows the sealing to be further improved, whilst at the same time ensuring the correct positioning of the abut- ment surface of the lead-in block with respect to the ap¬ erture in the lid. According to yet a further embodiment, a sealing material is located between the abutment surface and the inside of the lid. Using a sealing material between the abutment surface and the inside of the lid further im¬ proves the sealing. According to yet another preferred embodiment, the lead-in block is supported in said U-shaped housing by the bottom thereof, so as to counteract the force applied by the lid. Supporting the lead-in block directly on the bottom of the U-shaped housing is advantageous in that the bottom of the U-shaped housing is itself rigid enough to counteract the force transmitted from the lid through the lead-in block. Moreover, it provides a large area so that the resulting pressure from the force becomes low, and does not deform the lead-in block. According to a further preferred embodiment, the lead-in block is of a plastic material. This is a suit¬ able insulating material, which moreover makes the lead in block easy to manufacture, e.g. by injection moulding. The invention will now be explained in greater de¬ tail based on the schematic drawings, on which fig. 1 partially cut away illustrates the general windowpane assembly, fig. 2 illustrates a segment of the top box in the unassembled condition with the lead-in block in place but with the lid off, fig. 3 illustrates the segment of the top box of fig. 2 in the assembled condition with both the lead-in block and the lid in place, fig. 4 illustrates the top box with a Venetian blind, with the electrical connection plugged in, but with the lid removed for illustration purposes, and fig. 5 illustrates the top box of fig. 4 with the lid in place. Fig. 1 schematically shows the general construction of a multiple glazed windowpane assembly, viz. a double glazed windowpane assembly. In fig. 1 is illustrated a double glazed insulating windowpane assembly 1. The windowpane assembly 1 com¬ prises two windowpanes 2, 3, preferably made of glass. The two windowpanes 2, 3 are held apart by a spacer pro¬ file 4 and a top box 5. As will be explained below, the spacer profiles 4 are generally tubular and made of alu- minium. The top box 5 is also preferably of aluminium. Furthermore, the two windowpanes 2, 3 are adhered to the spacer profile 4 and the top box 5 in a sealed manner, e.g. by means of butyl. The two windowpanes 2, 3, the spacer profile 4 and the top box 5 thus form a sealed cavity 6. To further improve the sealing of the cavity 6, an additional sealing material 7 is provided between the panes on the outside of the spacer profile 4 and the top box 5 with respect to the cavity. This material could be butyl, silicone, or any other appropriate sealing mate- rial. In order to absorb any moist that may penetrate the additional sealing material 7, the spacer profile 4 is generally tubular and filled with a desiccant, such as silica gel. This prevents the moist from penetrating fur¬ ther into the cavity 6, where it is undesired. Integrated in the windowpane assembly 1, there is provided a blind 8. In the embodiment illustrated a Ve¬ netian blind, but the blind could be of any type commonly used, e.g. a roller blind, a pleated blind etc. The blind may be moved up and down within the cavity 6, by means of a drive mechanism located in the top box 5. The drive mechanism itself, which can best be seen in fig. 4, is not considered relevant for the present invention, and will not be discussed any further. As can best be seen in fig. 2, the top box com- prises two parts, a generally U-shaped housing part 51 and a lid part 52. The two housing parts may be secured to each other by appropriately interlocking mechanical parts. In the embodiment illustrated, the lid part com¬ prises barbs 53 adapted to cooperate with respective re- cesses 54 formed between two low flanges 55 and 56. Thus, the top box 5 may be closed, by snapping the lid 52 onto the U-shaped housing part 51. In this process the barb 53 resiliently slides over the low flange 55 and engages be¬ hind it in the recess 54, as best seen in fig. 3. Before closing the top box 5 by snapping the lid and the U-shaped housing part together, a lead-in block 10 according to the invention is placed between them. The lead-in block 10 comprises a socket 11 adapted to receive a plug 12 mounted on an electrical supply ca- ble 13. In the socket, contacts, preferably male contacts in the form of pins, are located. The contacts may how¬ ever also be female contacts. In fact, the constructional details of the plug 12 and socket 11 connection are not considered relevant. Such connections are well known for the skilled person, who will inter alia know that either of the plug 12 or socket 11 may comprise a male or a fe¬ male part. It should however be noted that the contacts are preferably Integral with the conductors leading through the lead-in. On the other side of the lead-in a second arrangement of plug and socket connector (not visible) is found. As there are no sealing problems on this side of the lead-in any known connector arrangement could be used. The lead-in block 10 further comprises an abutment surface 14 adapted to engage the inside of the lid 52, that is to say the side facing the interior of the top box 5 when closed. The overall dimensions of the abutment surface 14 are larger than those of an aperture in the form of a cut-out 15 in the lid 52, thereby allowing it to abut the inside of the lid 52 around the entire cir¬ cumference of the cut-out 15. In the preferred embodiment shown, a raised portion 16 is formed centrally on the abutment surface 14. This raised portion is adapted to protrude into the cut-out 15 in the lid 52, when the abutment surface 14 engages the inside of the lid 52. The dimensions of the raised por¬ tion are selected to match the dimensions of the cut-out 15, thereby ensuring that the abutment surface 14 is cor¬ rectly placed around the cut-out 15. The height of the lead-in block 10 is preferably selected slightly larger than the distance from the bot¬ tom 57 of the U-shaped housing would be to the lid, if the lid 52 were simply snapped onto the U-shaped housing 51, without the lead-in block 10 being interposed. This slightly larger height of the lead-in block, however, al¬ lows the abutment surface 14 of the lead-in block 10 to exert a force on the inside of the lid 52. This force will be maintained as long as the top box 5 is closed, and maintain good sealing over time. However, to further improve the sealing, a sealing material such as butyl may be placed on the abutment surface 14 around the raised portion 16. This sealing material will then fill whatever gap might exist between the abutment surface 14 and the inside of the lid 52, e.g. due to manufacturing toler- ances. Alternatively, the height of the lead-in block 10 may not be selected slightly larger than the distance from the bottom 57 of the U-shaped housing 51 to the lid 52. In this case the sealing material would be used to fill the gap, and transmit the force from the lid 52 to the lead-in block 10. In either case, the sealing mate¬ rial may also fill any gap between the raised portion 16 and the edge of the cut-out 15. With the lead-in block 10 in place and the top box 5 closed, the plug 12 may be placed in the socket 11 as shown in fig 5. As explained above, the top box 5 forms part of the windowpane assembly according to fig. 1. Dur¬ ing assembly of the windowpane assembly a dummy plug (not shown) would be placed in the socket 11 in the lead-in block 10. This prevents the socket 11 from being filled with the sealing material 7 when the gap between the win- dowpanes 2, 3 is filled with the sealing material. After the windowpane assembly as such is fully assembled, the dummy plug would be removed and replaced with the actual electrical plug 12 connected with the electric cable 13. Alternatively, the lid 52 including the lead-in 10, the actual plug 12 placed therein, and the electric cable 13 may be covered with a layer of appropriate sealing ma¬ terial 7 filling the gap between the windowpanes 2, 3. The entire windowpane assembly 1 including the top box 5 and the electrical connection via the lead-in 10 would thus constitute a sealed unit. In the embodiment shown, the lead-in 10 rests on the bottom 57 of the U-shaped housing 51 for the neces¬ sary support in counteracting the force from the lid 52 on the abutment surface 14. The lead-in 10 could, how- ever, just as well be supported indirectly or by other parts of the U-shaped housing 51. Thus it could instead be supported by appropriate protrusions on the upright walls of the U-shape, e.g. by the flange 58. However, as can be seen, this is not the case in the embodiment il¬ lustrated. Rather in the embodiment illustrated, at least one part 59 of the lead-in placed under one of the flanges 58 in order to hold the lead-in 10 temporarily in place until the top box 5 is closed with the lid 52. Though the present invention relates to improved sealing in connection with the electrical connections to the top box 5, it should be noted that good sealing of the connections could be achieved otherwise. In particu¬ lar, if any other part of the spacer profile 4 is pro- vided as a two-part construction, i.e. a box with a lid, the lead-in according to the invention could be used there. Also, as the top box 5 is electrically insulated from the spacer profile 4, both of which are preferably of aluminium, by means of corner plugs 60, a lead-in could be omitted. Instead electrical leads could be sol¬ dered to the outside and the inside of the top box, thereby not breaching the integrity thereof. Current could then be passed directly through the aluminium wall of the top box 5. In the same sense leads could be sol¬ dered onto either side of the spacer profile 4, viz. one lead on the outside covered by the sealing material 7, and a second lead on the side facing the cavity 6. This would then leave the integrity of the spacer profile 4 undamaged, and thus not affect the sealing properties thereof. The second lead, insulated itself, would then pass through the bottom of the U-shaped housing 52, which is necessarily open to the cavity 6 anyway. Moreover, if the spacer profile 4 comprises several segments insulated from each other by corner plugs, the arrangement last de- scribed, could be used with two of the spacer profiles 4, instead of using the top box 5.