Michael Clifford
Thank you both.
I have checked the axle measurement over shoulders and have 16.0-16.1 mm.
The coupling rods are jointed, and I have loosened the centre crankpin nut a fraction in case that was the problem, but no improvement.
The back-to-back seems to be the problem and the ‘boss’ on the rear of the driver appears to be the root cause.
Unfortunately I have had the wheels for some time and don’t recall from whom I bought them. In any case it’s not their problem so will try again to contact Markits direct, but that’s not easy as there is rarely any answer to phone or email.
If there is any progress I will update this thread.
Best wishes for the New Year
Mike