It has a DHW diaphram moving a pin which hits a flow switch. My best guess is that the pin isn't returning fast enough leaving the boiler fired up like there is still a demand for hot water but nowhere for the heat being generated to go. Run a tap and see when you switch it off how long it takes for the boiler to go out.
If it takes a while, that is most likely to be the problem and a replacement diaphram and some silicone grease to the pin should sort the problem.
Iain, January 2009
