I visited the UK's largest on-shore wind farm this afternoon. Some of
the turbines were running and others were not. I assume this was
simply a factor of demand.

There was one turbine we watched (turbine 40). It was running as we
arrived, then it stopped, then it restarted (all as others in the same
section continued to run). Why would they stop and start one turbine?
Do they need to be rested or tested or rebooted periodically?

How do they maintain the frequency if the rotation speed can vary with
the wind? I know they can adjust the angle of the blades but it was
clear the turbines were not all running at the same speed.