You obviously know your stuff. Thanks a million. It has indeed purged
itself and we now have hot water once more. Thanks for the last tip
too; I'll keep it in mind for future mishaps.

May I pick your brain a little more? Because of the awkward situation
of F&E tank I cannot access it in such a way as to be able to reach the
base to check depth. Is there a way to monitor this from outside? I'm
thinking of something like 2 'U' tubes fastened together in such a way
that one is inverted over the rim of the tank, reaching almost to the
bottom, connected to the other which extends below the tank base and
rises again to the rim. If I then syphon some water from the tank it
should find its own level and provide a water gauge. Does that sound
feasible?

If I'm pushing my luck with that question, please ignore it. I'll just
have to experiment.

Tony.