Vokera linear 28?

My boiler goes out on a 04 code I've had the switch and diaphragm replaced and its still doing it gets up to about 70 then the pin retracts in and the boiler goes off! If the switch is applied by the engineer manually by passing the pump it relights then goes off again! Any ideas I have a new pump ordered but not sure? oh and previously had the domestic hot water thermistor replaced Thanks