Time to investigate…
This in the rear brake internals. In addition to cleaning and lubricating, I smoothed some rough edges between the case and the guide pin (see arrow in photo) which was the main cause of the sticking problem.
The bike check revealed a damaged tyre (replaced), the front brake wasn’t great either (adjusted) and there was some play on the headset (tightened)
£35 charged in total (£13 of which was for the new tyre)