Peplace my 23 year old Baxi back boilor?

My 23 year old baxi back boiler has never given me a moments trouble, but to save gas I decided to get a quote from BG to replce it with Baxis new modern back boiler. Cost of parts £1500, old boiler out, new boiler in, new flue liner, one and a half days work. BG quote £5038 Why !!! for one and a half days work. Think my old boiler can stay for some time yet.