Electramate 'A' class please help!

Hi please can somebody help!!

I have a Gledhill electramate A class.
Serial number 9000000029
model number: AEA4270

story goes, nearly 4 MONTHS ago went away for the weekend came back to my flat all power was switch off (tripped out).
So switched back on turning boiler on.
boiler started up then after 2 mins, on the panel the light flashes = "overheat safety trip".
Codes= F.c , then F.|_ then F|_.
All motors seem to be working fine, (slight humming)
it keeps loosing pressure after a while (i keep topping it up)

I have had several engineers round including a few friends who are heating engineers and all of them have scratched there heads at it, most of them speaking to gledhill technical services. I have changed a the bottom heat sensor, as this apparently was a possibility, no joy.
Really do not no what to do, 4 months without hot water. Just want it working for christmas.

Any feed back with be greatfull
Thanks
Dan living in the Bournemouth Area, Dorset.