Hello Antony, I am not familiar with your type of control lifted from an invalidity carriage, but if the motor turns to power the car in one direction but not the other, then the problem lies with the reversing changeover relay.
What you can hear I think, is the relay pulling in when the power is applied, this means either one of the leads has parted from the relay connection, or the relay contacts are burnt out.
Your first point of call is a close look inside the speed control. If you can operate the control with the cover off, you may be able to identify the faulty component by touching the tops of the relay while oprating the control switch, you should feel the relay that's clicking. Once you have identyfied the relay that is operating but not providing any movement, look for for a displaced wire. If there is no obvious disconnected wire, if you have a multimeter, set to DC volts and put onto the heavy wire terminals and operate the controls again you should be getting a voltage on the terminals.
Regards,
Brian.