Just a follow up to an old thread (Google groups won't let me post to
a thread more than one month old) - http://tinyurl.com/5ohxs

Thanks for all your help.

My father in law and I replaced our old hot water cylinder as the
tests you suggested confirmed that a dodgy coil was to blame. I was
even able to use the cylinder that he had taken out of his house to
have a solar hot water system installed so it cost next to nothing.

So far everything seems to be working satisfactorily.