When I ran the hot water, I would briefly get hot water before it went cold and stayed cold. A plumber had a go at fixing it, charged me, and failed, so I resorted to internet research. This web forum was fab because I found a post from 2007 that sounded very much the same problem. With a bit of reading of posts & a good look at the boiler's online manual, it sounded like the diaphragm in the DHM Differential Valve was broken so I gave it a go fixing it myself. Never done this before, but it was dead easy. I ordered the diaphragm from AK Gas Spares (v.good quick delivery even on Royal Mail. I would recommend them). Also ordered a microswitch kit as this had been broken (possibly by the plumber in his failed diagnosis) As it turns out the Microswitch cases are designed to house two switches, so by relocating it in the other side, you can actually recondition one of these with some minor soldering. Anyway, followed the instructions from the online manual for replacing the DHW Pressure Differential Valve Diaphragm. It worked perfectly. I was surprised how easy it was. If you do it, here are my tips: OBVIOUSLY turn of the power first! Undo the sensing pipe nut slowly with the hot water tap left on. This releases pressure and more water will come out of the system. When you undo the screws on the cover, keep pressure on the cover, as there is a sturdy spring underneath. Make sure you look at how the parts fit together inside the Valve as you take it off. When you have replaced the diaphragm, put the spring and pushrod into the cover, and hold the pushrod on the other side with a pair of pliers while you screw it all back together, this will stop the spring making it difficult to screw it all on. Good luck to anyone else who tries this, and thank you to this forum for those who post useful advice! 
