Worchester junior 28i hot water no heating 5 flashes in service mode?

I have a Worchester junior 28i boiler, the hot water is working but not the heating and the green burner indicator light is off. I have repreasurised it and re-couples the thermostat device. When I put it into service mode the first time the green light came on momentarily and it powered up but then it stopped and flashed 5 times. I found a post on here saying i needed to defrost the pipes - seems unlikey as its not been cold enough, tried anyway but no help. The service manual says 5 flashes "service mode set to max press service button to return to normal" anyone got any ideas? or do i need to call someone out?