A couple of times recently, the hot taps have been running cold, and
the combi boiler doesn't fire up when a hot tap is turned on in the
house.

I've also noticed a lot of air pockets blasting and sputtering from
the hot taps when thy are first turned on. The unheated water from the
the taps seems to be at a pressure high enough that would normally
cause the boiler to fire up - but the water is coming out ice cold.

The central heating is working fine.

The boiler is a Britony Combi 80 made by Chaffoteaux et Maury.

Anyone tell me what the problem is?

Many thanks,

Dave L