Just to give the guy his due, it only took a few minutes to change the 3 port valve, which he first diagnosed as the problem. It wasn't, which I knew anyway. The rest of the time was spent looking for a fault in the circulation, even though the pump was brand new (fitted by yours truly) and showed a circulation of 1.1 cubic mtrs. Oh, and head scratching of course. Real problem was that even after all that time, the fault was not diagnosed. I was a bit peeved because, as already mentioned, I don't use the gas for hot water and, if I needed it, I could have replaced the valve myself for a fraction of the cost.