It's probably just some small part in the boiler that has gone wrong or something that needs adjusting but, unless you can find someone to fix things for a couple of hundred pounds, you may be best replacing the boiler. A new boiler will cost around £600 and it will be the cost of an afternoon's labour to fit it. You can choose your boiler on a site like screwfix.com (perhaps after talking to the engineer) and find an engineer on trustcorgi.com.
It's worth checking your bills and meter readings to be sure that the high bill isn't due to an estimated reading at some point.