Thanks Rich, so far I have had some helpful suggestions and I have been through the starting circuit and cleaned all the terminals and connections, stripped the starter motor and cleaned the commutator (no sign of bad connections anywhere, and the starter windings look good) and have put it all back together. I haven't had a starting issue since, but it's winter here now and I probably won't get high ambients until after December (daytime temps are usually in the low to mid 20s in winter). I have run an earth bridge from the starter relay (it is normally earthed via the ECM) so if I do have the problem again I can bypass the ECM and should then be able to crank even below the threshold voltage - if cranking is sluggish, then it has to be the battery. I'll update if anything significant happens.