gasgasnut

I have recently purchased a very well looked after txt 2000 andorra, i'm currently going through it, but struggling to get a service manual for it so i can work out how the speedo works. I have replaced the magnet on the rear disc and the batteries in the unit. it has the original gas gas barrell speedo which works but the speed is not being registered. if anyone knows where i could get a manual from or any clues what could be the problem it would be greatly appreciated.