In message ,
newshound writes

My guess too, classic airlock problem with vented gravity systems.

That is my only conclusion too, although I tried the suggestions by
Richard and Mike, holding my thumb over a mixer tap, and running a short
length of pipe from one tap to another. The second time I must have
left the cold forcing itself through the hot system for several minutes,
yet these guys did something yesterday which cleared it immediately.

--
Graeme