With no answers I've had a closer look and it seems like the problem is in the rear gearbox/diff? that sits inside the frame next to the rear left wheel. I can hear something slipping/missing inside when I turn the left wheel- but the right one is fine. Gearbox is made in france but I can't make out the brand.

I'd take it out- if I could work out how to get the rear wheel drive off. I can get the wheels off fine- but then what? I can see a spring on either side that appears to hold the whole mech up?

The springs look like what I call "sproings"- because they'll go sproing and vanish if I try and remove them