Continuing the saga with behavioural anomalies..
I replaced my LED lamp with a "quieter" EMI producing LED lamp. That helped. My UC300eth is powered by 5V from a USB cable. I moved that to a USB battery pack to reduce ground loops. Just learning about those! That helped massively with spindle erraticness. EMI is still present but its effects on the spindle are very minor.
So over the last few months I've been experiencing unexplained inaccuracies in machining I've put down to bad workholding and user error. Tolerances haven't needed to be critical, up until now. I started machining some small parts out of mild steel using two passes.