It stopped working, suddenly, about a week ago. At that point the
header tank (5 gall) seemed to be empty or, at least, too low for me
to reach its base (it's rather inaccessible). I replenished it to about
2/3 capacity.

Have just checked the tap feeding from it. It wasn't fully open but is
now.

The airlock sounds a likely possibility. Can it be cleared without
disconnecting anything?

I'll run the system again now to see if it clears itself but, if not,
I'll wait for your further advice.

Many thanks for your help so far.

Tony.