Nigel, I didn't read the second page of this topic, hence this reply.
Electric master clocks as you probably aware from of my other posts, i collect them and know a bit about them too.
The power station masterclock had a differential mechanism in it which was driven via a mains operated clock and a pendulum precision clock,
( now these days the pendulum clock is a precision quartz clock which may even get corrected occasionaly via a satelite time signal when the drift is out more than 1 second or so. )
If the difference was say more than one or a couple of seconds slowly the governor is adjusted by a minute amount. This has to be done slow otherwise the risk exists that one or more power station alternators grab all the load and trips on overload. Reactors are fitted in the lines to the step up 11 kV / 220 kV transformers to dampen excess currents, also the transmission lines will absorb minor frequency differences in extra losses.
A thing you want to avoid in a grid is hunting of alternators which can cause instability and stations to trip out on over or underfrequency. Time delays are also applied to the controlgear to minimise it going back and forth.
Alternators, when synchronised will keep in synch with each other. If the prime mover loses power the alternator will run as a motor and will remain locked in untill taken off line by the protection settings.