Hello, I have a problem with my Sime Friendly boiler - the heating is working fine but the hot water will only come on when the heating's on full blast. When I turn on the hot taps it doesn't do anything, no attempt to fire up at all. My plumber has been out and replaced the diverter valve diaphragm but that hasn't fixed the problem - can anyone help?

Lil-P, September 2014