I've had the plumber in several times since switching the heating back on this year, had a new thermocouple installed and everything.
Finally he pointed out that there should always be one radiator in the system which is deliberately left without a thermostat. Otherwise when the weather temporarily warms up again they all switch off and the boiler overheats, triggering an automatic cutoff. Mine was switched off, and since I've switched it back on the heating has worked fine.
Check which one yours is, and make sure it's left switched on.