All Mod Cons

Hi - here's the update to anyone who may have similar symptoms.....

I changed the hot water tank and the problem is solved !!! Hoorah !!

Yes, I guess as I thought and some people suggested, the coil must have had a split causiing a leak between the two systems. When I took the old tank out it had loads of limescale, some of the bigger chunks were pipe shaped - as if they had formed on the coil and then dropped off. Some were quite thick.

I hope this information helps others with similar problems. Thanks again to all who posted replies.
Richard