In response to my own posting and hopefully of some use to anybody else experiencing same problem. Vokera engineer arrived this morning, removed front from boiler. Replaced a part which evidently monitors the temperature,which was giving a false reading due to it being faulty. The engineer replaced this part ,which seemed to simply pull out. Turned on the boiler, works perfect now. Viewing the boiler from the left hand side, the part replaced was covered in a rubber sheath, located at the upper right of the side panel.