I've just realised what you meant and wiggled the thermostat on the cylinder. I then switched on the hot water and the boiler miraculously sprang into life. I don't know why, I'm just grateful that it has. Thanks for your help, I've learned a little bit about hot water systems from this...
Sorry too technical for me. What's a cylinder stat? There's a pipe with a tap going in to the top of the hot water cylinder. When I switch on the hot water on the control unit, the power light comes on but nothing else happens i.e. the boiler does not fire.
My central heating is fine but although the light comes on on the control panel when I select hot water, the boiler does not fire up and water is not heated. It's a Potterton Netaheat (circa 1985!) and a Siemens non-digital control unit which I bought to replace the original less than a year ago