Just had the same problem, it was the diapham for the hot water switch, diapham had perished, system is 6 years old. cost was £31 for part. It's very awkward to do but possible, you need to remove the switch by pulling out the clip then remove the 8 bolts on the housing of the diaphram. it is very awkward to get screwdrivers in, I managed it with offset screwdriver. The housing pulls down from the connection above, please be aware which way the diaphram goes back in
Spike, April 2007
