Turret slave dials used to have a poor reputation for reliability. This may simply have been caused by moisture running down the hands to freeze in the hour pipe. There is no reserve of energy to move the hands beyond the normal return spring. This offers a degree of safety since no energy is consumed trying to work against a blockage in the mechanism.
The hands stopped again overnight but restarted on their own. Leaving the hands at completely the wrong time! So I ran some light 'cycle' oil into the hour pipe this time. Just to see if that helps and then reset it to time. A clock dial showing the wrong time is far worse than one set to 12 o'clock and deliberately stopped. Mains synchronous clocks used to to be non-self starting in case they fooled the owner after a stoppage. Better to see the hands have stopped to prove the wrong time is being shown.
The clock stopped a couple of times more after snow and overnight frost. Reinforcing the theory of ice being trapped between the minute arbor and the hour pipe. I applied yet more oil and slave worked again despite -7C overnight frosts. Even more oddly, the dial gained the 20 minutes it had lost and continues to show the correct time after two days. Well, that saved me getting the ladder out again but makes absolutely no sense.