I was apprehensive too but it appears to have done the trick. For now I just rigged up a relay to test the theory and tested it about 8 times. Each time the starter engaged perfectly without any clicking / delay etc. Still need a lot more testing but so far it looks promising. Normally it would have played up by the 3rd or 4th try.
Still undecided whether to set up a permanent relay (another failure point and still relying on 20 year old electrics) or run a new wire with a start button because race car.