Worcester Bosch 1924 CBI Boiler Problem?

Hello there
I have a Worcester Bosch 1924 CBI boiler which is about 5 years old. We have just had it serviced by a local tradesman who didn't know what to do with it! It has just started to turn itself on and off and then it shuts itself down and repeatedly flashes twice every second or so. It's not cold outside so there's no way of it been a frozen pipe so does anyone have any ideas or experiences of this type of boiler going wrong? We have reset it but it's still going back to the same fault. Any feedback is highly appreciated. Many Thanks