Hi, a bit late I know, but for future reference....I had this problem, it was a faulty valve in the airing cupboard. Its one of the electrically controlled ones, it wasn't opening and so the hot water wasn't able to get into the tank, the boiled was just heating a tiny bit it water in the pipe and so it was thinking it was up to temperature after a few seconds. I replaces the value (just the motor part on top, not the acutal pipe part) and hey presto it worked perfectly.
hth
G
Cyril the Plumber
October 2009
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