If your system is open vented then check the feed & expansion tank (probably
in the loft). The float valve may have stuck causing the tank to run dry, a
few yanks up and down usually frees it and gets the water running again.
Alternatively if the tank is full then check that the cold feed is not
blocked where it tees into the system.

Thanks for that - we've already drained and refilled it a few times
recently while decorating, and everything seems to work as normal
though :-}