I don't think EnergyWatch/Consumer Focus will be of much help here as its not the gas or electricty side of things at issue here, although they may be able to apply some pressure.
If they are now stating they are unable to repair/cover your system i would be inclined to ask for the date at which this became policy and for all payments since this date to be refunded in full. As said previously if they have been inspecting your boiler and taking payments they have accepted liability to maintain the contract.
This is of course if you are unable to exert sufficient pressure for them to repair your boiler regardless.
In addition many other suppliers alsooffer a similar servie, E.on I bleeive now matches BGAS's portfolio of extra's covering electrics, plumbing drainage as well as the boiler, may be worth a look for on-going insurance.