Great news! I think I've cracked it. Thank you to everyone for your help and advice.
I'm a little embarrassed to share HOW it's fixed but...
Wrong screw in the wrong hole.
The tt01 chassis is getting on a bit now. I got it second hand and it has a range of screws (half of them are not factory and the other half don't tighten anymore as the threads are gone). One of these screws is about 2-3mm longer than all the rest. It's a bit of lottery which ones go back into which holes once I've pulled it apart. This longer screw one the lottery the last couple of times I've had it apart to check everything. It's just long enough to go through the plastic gearing cover and touch the largest cog.
It did feel like something electronic rather than mechanical. I was wrong.
Thanks again for the help here on Rctech. It's a great forum.
It's mid night now and thunder storms outside (New Zealand). I'll be taking it into work tomorrow to have a lunch time trial run. Fingers crossed that was the only problem.