If your boiler is currently working ok then take something up with the boilers manufacturer or someone recommended and local. If you have a serious problem then sign up to BG for a year.
The British Gas contract will even take you on while your heating system/bolier is totally fooked, all you have to do is sign up for a minimum of one year. After that year cancel and go elsewhere. They will repair any fault and be with you the day you make the call to sign up. First time I have ever experienced a maintenance contract which you can apply to something which is already fooked. My boiler needed two new sensors, a service and a PCB. My Dad is a heating engineer and even he said sign up to BG for a year as the total monthy outlay for the year worked out at less than the price of the parts, plus I now have 12 months cover.
