halstead ace high boiler?

Hi after reading some of the posts on this website which were having the same problem as me (boiler playing up firing and then stopping resulting in hot cold hot showers !) it was suggested that the diaphragm of the flow switch could be damaged so my husband replaced it. The heating engineer I had out also said it could be the heat exchanger had scale so he removed it and cleaned it. Started ok had hot water yey... then tried the shower and it tripped. Reset the boiler and it tripped again and the power light is flashing. Any ideas would be very gratefully received as I have a baby who I need to clean up after and the kettle is taking a battering!