I've just remembered, i have a spare transmitter and receiver pair here at my flat. I could bring that along on Sunday, and quickerr replace the receiver in the bot and see if it has the same drive issues. If it does, it would point to either something in the ESC, a motor behaving not quite identically as the other, or a few other things. But it'd certainly eliminate the transmitter/receiver if this works fine - and we'll have a pinpoint to help wittle down the issues in those two if we need to stick to the transmitter/receiver pair we originally used.

The one I have is a fairly basic one, buts it has six channels - it was only ever intended for featherweight and below stuff i was doing at home at the time - https://www.ebay.co.uk/itm/381958044606