Thanks Headrush. I have been away for a few days, and not looked back at the system as have the immersion heater. I have two meaning one each for hot water and heating respectively. I have opened the manual lever and nothing happens (thought it might fire up). Also when I switch the central heating on, the hot water still doesn't heat up.
Headrush wrote on Aug 10th, 2016, 11:43pm: More often than not, this is a sign that an electronic valve has failed. There may be two of these devices on seperate circuits or a single 3 port device controlling both HW&CH, usually located near the pump. These valves usually have a manual lever on the side which can be pushed across and latched into place forcing them to the open position. If you switch on the HW only and manually set the valve, the system should fire up but sometimes the microswitches inside the valves are at fault, in which case it won't work unless the CH is switched on too, as this appears to be working.