This invention relates to game playing apparatus for playing a game involving random selection of numbers and particularly to an electronically operated roulette simulator. The game of roulette, as played in casinos with mechanical apparatus, comprises placing wagers on which of thirty six or thirty seven spaces or numbered sockets, a ball, projected from an impelling apparatus will finish after traversing a bowl several times, until it looses energy and settles. Sockets are conventionally numbered from 1 to 36, upon which wagers are to be placed, and it is common practice to provide a thirty seventh "zero" space which represents the house stake. Alternate numbers are also picked out on red or black, and it is possible to wager on the number or colour upon which the ball will settle, with the pay out related to the odds - correct predictions of a number may result in a 36: 1 payout, whilst, a correct prediction of a colour only yields 2: 1 . The "house" wins if the ball rests on the zero, or on unclaimed number or colour. It is well known to provide coin or token operated game playing machines which simulate roulette. These generally follow the pattern of wagers and pay-outs established in the casino game. Legislation however limits the maximum stake (50 pence) which can be placed in any one game, and with a maximum payment of only 36 times the stake, other games which offer larger maximum payments are proving more popular, even if the chances of winning are reduced with the higher payout: stake ratio. It has recently been proposed to provide a coin or token operated game playing apparatus wherein it is possible to simulate the playing of two or more, up to e.g. six simultaneous roulette games, each of which takes place on a separate simulator. An object of the invention is to provide a coin, token or otherwise operated game playing apparatus simulating roulette based game, in which provision is made for the multiplication of odds, and therefore the payment/stake ratio. According to the invention a game playing apparatus is provided which is adapted to simulate a game of roulette, characterised in that means are provided whereby the use of two or more balls in simultaneous or consecutive play may be simulated in conjunction with a simulation representing a single roulette wheel. Simultaneous play means that the balls are projected together or rapidly one after the other before previous balls have come to rest. Consecutive play means that each ball is projected immediately when the preceding ball has come to rest, to give in effect a plurality of games for a single play operation. This multiple - play feature means that an increased number of combinations can be the subject of wager, and that also the stake can be increased up to say six times the legal maximum single stake where six simulated balls are used. Two or more simulated balls, up to any practical limit may be used. Options include, staking all in a single number, so that the odds against winning, and hence against the maximum pay out are multiplied by 36n (wherein n = number of balls used in a single play sequence) whilst the pay-out itself is multiplied by 36n2 . Thus, whereas with a maximum 5Op stake, at 36: 1 the maximum payout on a single ball game Is £18.00 with six simulated balls, the maximum payment in the event of all balls finishing on the nominated number, (odds 216: 1 ), the maximum payout, with a permitted stake of £3.00 is £648. Further options are nominating a different number for each 50p stake, so that the odds are reduced to say 6:1 with a £18.00 pay out on a £3.00 stake, or a combination of a colour with one or more selected numbers (up to a reduced maximum). The apparatus according to the invention therefore preferably includes means for simulating a roulette wheel, means for displaying the simulated roulette wheel, means for simulating projection of a plurality of balls into the simulated roulette wheel simultaneously or consecutively, means for simulating the trajectory of each said ball over the simulated roulette wheel, means for bringing each simulated ball to rest after a random time interval at a randomly determined place on the simulated roulette wheel, and means for displaying the simulated motion of said simulated balls over said simulated roulette wheel. The game playing machine may include a display panel including a display simulating a roulette wheel, and showing the simulated path of each ball, and may also include selection means for choosing one or more numbers, and for selecting options such as red or black or odd or even numbers, and also a slot for insertion of stake money a start button, win or lose indicating means and a slot for paying out prize winnings. The machine may be operated by coins, or tokens, and pay out prizes in cash, tokens or goods. A game playing machine according to the invention will now be described by way of example with reference to the accompanying drawings, wherein:- Figure 1 is an elevation view of an embodiment of game playing machine for simulating a roulette game, in accordance with the invention, and Figure 2 is a block diagram summarising by way of example the operation of a game playing machine such as shown in Figure 1 . A game playing machine according to the invention for simulating a roulette game, has a front panel 10 having an upper section 1 1 , with a display 12 simulating a roulette wheel, having a central raised portion 13 and a peripheral groove 14 divided into 37 compartments numbered 0-36, and apart from the zero, coloured alternate red and black. The display 12 is adapted to simulate the simultaneous or consecutive projection of a series from 2 to for example 6, of simulated roulette balls into the wheel, to each circulate there for a randomly determined time interval and to then come to rest on a randomly selected one of the compartments 0-36. The simulated balls and their trajectory are generated with random parameters by computer means, and displayed through the intermediary of a display driver. The number of compartments may be increased, for example to indicate OO' and 000. The front panel 10 also has a middle section 15 which incorporates a coin insertion slot 16, a push button matrix 17 for selection of wagers, and a row of operating buttons 18. The matrix 17 includes number selection buttons from one to 36 and four option selection buttons for selecting red or black, or odd or even numbers for different possible wager combinations. A lower section 19 of the front panel includes a pay-out slot 20 for paying out any prize won as a result of a successful wager. Figure 2 illustrates a mode of operation in block flow diagram form. One or more numbers up to a prescribed limit are selected by pressing appropriate buttons or pressure sensitive areas in the selection matrix 17, and/or options are selected using the E R B or D buttons or buttons 18, and appropriate coins for the bets placed inserted in slot 16. The selections are transmitted to a setting circuit - 21 which stores the selected values, while a sensor associated with the coin slot 16 signals to a start means 22 the value of stake placed in the slot 16. When insertion of coins is confirmed and a start button pressed, the start means 22 triggers the timers 1-6, 23 and CPU 24. The latter computes the virtual trajectory of each of the simulated roulette balls, terminating each trajectory separately when the respective timer is stopped by a randomised stop signal from a randomiser 25. The computer 24 converts the resultant stop position of each ball to a corresponding number, and also determines whether each number is red or black, odd or even and applies the result to comparator 26 where it is compared with the settings from the setting circuit 21 which correspond with the player's selections. The computer 24 also drives the display 12 to give a visual display corresponding to the computed trajectories and the eventual result. Comparator 26 determines to what extent the settings and the result correspond in accordance with rules established for each type of wager, and determines whether the player has won or lost, triggering "win" enabler 27 to make an appropriate payout of coins or tokens from slot 20, or a "lose" enabler 28 to drive on appropriate display, and credit the "house" or "bank" which may involve physically transferring coins and tokens to a separate storage container. It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention. For example, the apparatus may be adapted to accept stakes, and pay out prizes in any suitable manner, that is by coin, bank note, token, or credit card, and the apparatus may be adapted to be played on-line, with a virtual panel displayed on the player's computer screen rather than a physical panel as in an arcade version, with click-on features replacing push buttons but retaining a similar display of the roulette wheel and simulated trajectories of the multiple balls. Any number of balls, from 2 up to any practical or desirable limit may be used in playing of the game.