It sounds like he was telling u porkie pies, there are no exclusions if u dont get a spiro fitted, but if the water becomes sludged up then you'll get a quote 2 have a powerflush done which is 600 quid, the spiro is fitted to help prevent that.
Listem mate if the pressure is continuously rising and ur not adding any more water to it yourself then the fill loop or twin pass heat exchanger are passing. Disconnect the fill loop if its still doing it then its the heat exchanger passing. The expansion vessel is not faulty.
Call a qualified gas safe registered engineer out,no one will give you any advice on here on how to cure your fault. Sorry mate time to get yournwallet out again!!
Bg dont sub contract repair work, they get subbies to do the services. One call out a year pays its self and you get your boiler serviced and unlimited callouts. Here we go again with the bg bashing.
Get british gas out mate, one off fixed price repair and they probably will ADVISE you to change the boiler, but they'll still fix the boiler. Its up to you if you want to change your boiler, only ADVICE.
I think your problem might be that your getting plumbers out to fault find on a boiler and unless your qualified then your breaking the law opening up the combustion chamber on your boiler...pay the money ang get someone whos got a clue about boilers, especially yours as its one of the easiest...
It sounds to me like you should get someone in that knows what they're doing and stop opening up the combustion chamber.(unless your qualified to do so)
Not a cocky answer mate, just saying that if a pcb on a compact needs changed then it has to be done by a registered gas engineer as there are adjustments that need to be made that only a qualified person can do.