A BALL-ACTIVATED TOOL FOR USE IN A DRILL STRING This invention relates to a ball-activated tool for use in a drill string. As is well known in the extraction of sub-surface liquids and gases from underground reservoirs, e.g. in the extraction of liquid and gaseous hydrocarbons, it is usual to employ a drill string and related tools, and such technology is well known to those of ordinary skill in the art and does not require detailed description herein. The ball-activated tool of the invention has been developed primarily in connection with the activation of hydraulic tools, such as "under reamers". At present, it is usual to employ a so-called "pressure-up sub" (PUS), and which relies upon use of shearable pins in order to activate a related drill string tool. However, the use of shear pins introduces an element of unpredictability, in that typically shear pins may vary from their notional design shear fail loading by significant amounts, and this can result in a reduction in reliability, so that so-called "activation consistency" can be of the order of plus or minus 15%. The present invention therefore provides a different type of tool which utilises fluid pressure to activate related drill string tools, such as an under reamer, but which does not rely upon the loading of shear pins for activation. According to the invention there is provided a ball-activated tool for use in a drill string in order to activate a related hydraulically operated device, such as an under reamer, and which comprises: a hollow main body adapted for mounting in a drill string and through which fluid can flow when the tool is an a de-activated mode; a tubular collet slidably mounted in the main body for movement between a retained inactive position and a released position corresponding respectively to the de- activated mode of the tool and an activated mode; a ball-receiving seat coupled with the collet and arranged to receive an activating ball launched from the surface and down the drill string to activate the tool; spring means arranged in the main body to maintain the collet in the retained position; a retainer arranged in the main body to engage with and to retain the tubular collet in the inactive position, and to release the collet when the tool is activated; an activating sleeve coupled with the collet for movement therewith to an activating position of engagement with a stop on the main body; a first by-pass port provided in the collet and communicable internally with the interior of the collet and externally with the space defined between the outer surface of the collet and the inner surface of the main body when the tool is activated; and a second by-pass port provided in the activating sleeve and communicable externally with the space defined between the outer surface of the sleeve and the inner surface of the main body, and internally with the interior of the sleeve, when the sleeve reaches its activating position; whereby, upon engagement of an activating ball with the seat to activate the tool, the following sequence takes place: a. fluid pressure builds-up upstream of the seat; b. subsequent release of the collet by the retainer; c. movement of the collet, the ball and the seat, and the activating sleeve until the sleeve reaches its activating position; and d. by-pass flow of fluid around the ball and valve seat via the first and second by-pass ports so that pressurised fluid can flow via the main body to activate the related hydraulically operated device. Conveniently, the activating sleeve is engageable with an internal shoulder provided on the main body to form said stop. The collet may be coupled with the activating sleeve for movement therewith via said valve seat. Conveniently, the ball-activating tool according to the invention is coupled with a related hydraulically operated device, and preferably an under reamer. The main body may therefore include a top sub in which the tool is incorporated, and a bottom sub in which the under reamer device is mounted. The under reamer includes one or more cutter movably mounted in the bottom sub for movement between a withdrawn inoperative position, and an outwardly projecting operative position. A flat spring arrangement may be provided to engage via its outer side with said cutter, and on its inner side is exposed to fluid flow through the main body when the tool is activated, such that the spring arrangement can operate to press the cutter outwardly to the operative position. Conveniently, the retainer and the spring means comprise an assembly of a retainer ring, a set of spacers and spring washers. The retainer ring is therefore preferably a rigid retaining ring, which pre-loads the spring washers and also retains the collet. The retainer ring holds the collet in place, and fluid dynamics will not affect it. A preferred embodiment of ball-activated tool according to the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a detail longitudinal sectional view of a ball-activated tool according to the invention, for use in a drill string, in order to activate a related hydraulically operated device, such as an under reamer, and showing the tool in a de-activated mode in which fluid flow through the main body of the device is permitted; Figure 2 is a similar view, but showing the adjustment of the components of the tool following launching of an activating ball from the surface down the drill string to activate the tool; Figure 3 shows, in the separate views a, b, c, d, thereof, fluid flow path relative to the tool in, respectively, (i) the deactivated mode of the tool, (ii) the launching of an activating ball to initiate activation of the tool, (iii) the build-up of fluid pressure on the activating ball after it has been received by a ball seat and to pressure-up the system, and (iv) adjustment of the components of the tool under the action of the pressure-build-up in order to activate the hydraulically operated device; Figure 4 is a longitudinal sectional view of a top sub and a bottom sub of a drill string, in which the ball-activated tool of the invention is mounted in the top sub, and a hydraulically activated downhole tool, such as an under reamer, can be mounted in the bottom sub, and the figure showing the ball-activated tool in its de-activated mode; and Figure 5 is a view, similar to Figure 4, but showing the ball-activated tool in its activated mode, in which it can route pressurised fluid to operate the related downhole tool (not shown in detail). Referring now to the drawings, a ball-operated activated tool according to the invention is designated generally by reference 10 and comprises a hollow main body 11 (forming a "top sub") which is adapted for mounting in a drill string in order to activate a related hydraulically operated device (shown in more detail in Figures 4 and 5). The device shown in Figures 4 and 5 is an under reamer, but it should be understood that this is merely one example of a related hydraulically operated device which can be activated by the ball-activated tool of the invention. The hollow main body 11 permits through flow of fluid to take place when the tool is in a de-activated mode, as shown in Figure 1. A tubular collet 12 is slidably mounted in the main body 11 for movement between a retained inactive position (as shown in Figure 1), and a released position (as shown in Figure 2) corresponding respectively to the de-activated mode of the tool and the activated mode. A ball-receiving seat 13 is coupled with the collet 12 and is arranged to receive an activating ball launched from the surface and down the drill string to activate the tool. A ball 14 is shown in Figure 2 in engagement with the seat 13, and with the tool components adjusted to a released active position, which causes activation of the tool. Spring means 15 in the form of a set of spring washers is arranged in the main body 11 and which act to maintain the collet 12 in the retained position shown in Figure 1. The spring means 15 cooperate with a retainer, in the form of a rigid retainer ring 16, and two end spacers 17, in order to retain the tubular collet 12 in the inactive position. However, upon activation of the tool, as will be described in more detail below, the collet 12 is released by the retainer ring 16, and against the opposition of the spring means 15, in order that the collet 12 can move to a released position which initiates adjustment of the tool to the activated mode. An activating sleeve 18 is coupled with the collet 12 for movement therewith to an activating position of engagement with a stop provided on the main body, as shown in Figure 2. hi the embodiment of Figure 2, the stop is provided by an internal shoulder 19 which limits the movement of the sleeve 18 and collet 12 to the active position shown in Figure 2. Although not shown in detail in Figures 1 and 2, a first by-pass port is provided in the collet 12, and which communicates internally with the interior of the collet and externally with the space defined between the outer surface of the collet 12 and the inner surface of the main body 11 when the tool is activated. This will be described in more detail below with reference to Figure 3. There is also a second by-pass port provided in the activating sleeve 18 (also not shown in Figures 1 and 2), and which communicates externally with the space defined between the outer surface of the sleeve 18 and the inner surface of the main body 11, and internally with the interior of the sleeve, when the sleeve reaches its activating position shown in Figure 2. Referring now to Figure 3, the four views a, b, c and d show successive stages of adjustment of the tool between the deactivated mode and the fully activated mode of the tool. hi Figure 3a, normal fluid flow down the drill string and through the interior of the main body 11 is permitted, and during this time the related hydraulically operated device (the under reamer) remains inoperative. Figure 3b shows initiation of adjustment of the tool to its activated mode, which is caused by launching activating ball 14 from the surface and down the drill string, to engage seat 13. Figure 3 c shows the components of the tool still in the deactivated positions, but with the ball 14 engaged with the seat 13, pressure builds-up upstream of the ball and pressures up the system until such time as the fluid pressure force acting on the ball 14 causes the collet 12 to be released by the retainer ring 16, so that the assembly of components 12, 13 and 18 move as a unit to the position shown in Figure 3d, such position being defined by inter-engagement between the outer end 20 of activating sleeve 18 with shoulder 19. As shown in Figure 3 c, the potential flow of fluid through the system, shown by arrow 21, is initially prevented by virtue of the seating of ball 14 on the seat 13, until such time as the pressure build-up is sufficient to cause the collet 12 to be released by the retaining ring 16. Figure 3d then shows the fluid flow path through the system, which is at a higher pressure than the through flow in the deactivated mode of Figure 3 a, and such pressure is sufficient to trigger operation of the under reamer. As can be seen in Figure 3d, the fluid flow effectively by-passes the ball 14 engaged with seat 13, by first passing outwardly from the interior of collet 12 through one or more first by-pass ports 21 to the space 22 between the outer surface of collet 12 and the inner surface of main body 11. The by-pass flow then returns to the interior of the main body 11 via one or more second by-pass ports 23 in the activating sleeve 18. This resumed through-flow of fluid, at enhanced pressure, and shown by arrow 24, then passes to a hydraulically operated downhole tool (preferably an under-reamer), arranged below the tool 10, to initiate operation of the latter. Figure 4 illustrates schematically a top sub 25 in which the tool 10 is mounted, and a bottom sub 26 in which a hydraulically operated tool (27), such as an under reamer can be mounted. Figure 4 shows the tool in its de-activated position, and Figure 5 is a similar view to Figure 4, but showing the tool 10 in its activated position in which it can route pressurised fluid to operate the tool 27 e.g. an under reamer.