I re-parametrized the new robot wheels and they turned out a lot better! I'm controlling the motors with an LM298N dual H-bridge driver. Cheap as chips these things, but with a few disadvantages, not the least being the 4V voltage drop across the H-bridge requiring a separate motor supply - since the working voltage of the DC motors I'm using is between 3 and 6V and the electronics runs at 5V.
Rotation sensors on the gearbox are proving to be challenge. There is not much room in there and the geartrain has too much backlash for a self-balancer really. Maybe I should have used servos adapted to continuous rotation - but they are sooo slooow!
This evening I'm popping round to a girlfriends to help with her computer, Microsoft's Skype disaster is causing problems. She is THE Chutney and Sweet Pickle Queen of the local Markets and I'm hoping to stock up at the same time. Cheese and Chutney - a marriage made in heaven!