On these there is 2 thermisters and they can be change in about 2 mins to check if faulty the one down by the plate heat ex is usually the faulty one and symtonsd are rapid on off after a few mins if it fails copmpletly it can be changed with the other one to get boiler up and running.
Personally i would say it is the board gone cost 70 quid plus vat so anything more than a ton for part and they are taking the P .
Themistor is less than a tenner
I was really weary when he said it was only the thermistor, it didnt sit right with the symptoms...plus he didnt want to listen.
What do the thermistors look like, could i change them round myself to get the heating going.
The PCB gone sounds not too far off the mark hence the continual on/off cycle that its going through.