£550 is a bit much to pay for an enineer's guess... it "MAY" need descaling.
Saturday failure is strange - never ever on a Sunday? Or any other day?
Reading people's experience with B Gas on here I am surprised they're quoting so little - I think they must get big bonuses for upselling unnecessary work. Were I you I would cancel my contract with B Gas and find a reliable Corgi engineer with a reputation to maintain - let's face it your service and maintenance contract has not stopped the Saturday fault.
You have my sympathy...
Peccavi
February 2009