shoprider sovereign mobility scooter?

my dad has one of these - when he switches it on, it will go forwards. If he tries to reverse, the beeper beeps, but it will not move. The beeper carries on, and if you put it in forward, it will now not move, but still beeps as if reversing. he has to switch the scooter off to cancel the beeping, and can then go forwards if he switches it on again - with the same fault repeating when he tries to reverse. I'm stumped!