@dc42 Problem solved, thank you!

I'd already checked for any slip or backlash by holding the belt while jogging small amounts, but was going to double-check the grub screws to be certain.
They need a torch to see them clearly.

They are OK - but I was not expecting the pulley to have changed size!
A small fragment of fibre reinforced tape from somewhere was stuck on it, so the belt was moving slightly further for each motor rotation.

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