We are indeed on the same page Colin, I had that sort of relay in mind though a solenoid might be better for higher amperage peaks? I have a circuit diagram made up for this but I am going to see if I can simplify it down to two relays and four diodes rather than four and eight respectively before buying anything.

Need to get cracking through, the A28-400 motor arrived yesterday. Just waiting on some gearbox parts before assembly.