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Wheels are up against the nylon? bearings but no tighter than the others. No side to side movement in the back axle. 
Backlash? Not sure how to judge or measure that but as mentioned, the gear mesh feels tighter with no way to adjust it.
Front tyres are not coated but are glued and trued, as they are on the other six.
(You might have hit on something here though.   It looks like the right hand front wheel/tyre is off the track. Strange as it WAS fine when originally set up!)
Rear tyre dia. 21.00mm and 21.01mm

I'll strip the car down again and check nothing is binding and the chassis is flat. I suspect not, since the rolling chassis exhibited the same behaviour.

Thanks guys.

I'll report back when I've finished investigating, probably tomorrow.