Permanent F5 fault?

I have a Glowworm 18hxi boiler with a Systemate 2000 Gledhill system. Boiler keeps failing with F5 fault. Have had plumber out six times - changed Thermistor and overheat thermostat on flow and return plus new printed circuit board on boiler. Two new pumps on central heating and hot water in Gledhill. Thermostat in hall will click when you turn down but not when you turn up! When reset button set radiators heat up for about 30 mins then fault occurs again. Any other solutions?