I managed to build a radio control transmitter and receiver on Saturday night, attached it to the small prototype and woohoo – one side worked! The next morning I was able to locate the problem, replace one of the integrated circuits and now both sides work, with forwards and reverse… sort of:

The green light on the receiver shows that it has a connection to the transmitter. If the light goes off then it means it has lost its connection. I’m not sure why it’s losing connection at the moment but I would hazard a guess that the motors are drawing too much power and there’s not enough left for the receiver, so it’s thinking caps on and back to the drawing board for this project.