Strange behaviour x and y axis - corner of print
After one further check (stepper driver current/VREF), which I didn't think could be the problem due to not adjusting to cause the problem; it turns out this actually was the problem.
All stepper drivers have been reset back to correct VREF for their respective motors minus 10% as a safety margin and everything is printing well again.
I have no idea how VREF could have changed by itself, however thermal expansion on the boards due to constant use may be an explanation.
For anyone else experiencing similar issues and who have discounted the stepper drivers as the problem, I suggest resetting VREF to see if that helps.