Thanks to everyone for the help - I really appreciate it.

Turning the pump down seems to have cured the problem - no more air.

I'll put some anti-corrosion stuff in tomorrow and keep the suggestion
of flushing in the back of my mind for the Summer hols.

This whole house really is a botch-job. I noticed when we first moved
in that using the microwave and electric oven together caused the
lights to *really* dim. Checking it out, I found the whole kitchen
(including electric oven, sockets and lights) is run from a single
piece of 2.5mm standard ring cable straight from the fuse box. Arrgh!

Thanks again,
Rick.