no hot water from potterton 80E?

Returned from holiday a month ago and potterton netaheat profile 80E has stopped producing hot water. Central heating working fine. In hot water mode the system fires up fine when cold for about 5 minutes and then cuts out and then cycles on and off but stays off more than on and so water does not get warm/
Pump is fine, valves fine (been changed) and thermostats in boiler all changed. Made no difference

Is this a water flow problem - header tank is full and OK in the loft and pipes are hot from boiler. Does the system need flushing? Is there an airlock? Is the thermostat on the water tank a problem? Is it the controller board?