If it’s just once so often, it’s probably firing up to heat the small internal hot water tank. Mine seems to fire up about once an hour say, and yes when it does this it also runs the central heating pump.
In my previous house, the combi could be selected as to whether to preheat that small internal tank or not. The boiler in this house just does it.
You could have a read at the owner’s manual – or phone the manufactures help line.
Plumbers should still be working – but yeah, I know can be hard to get hold of at any time.