If you pay Homeserve £60 a year, they need to take their ginormous expenses and profits out of that, so how much does that leave to pay for your boiler service? £25? You'll get some bloke who has to fit in 10 or so calls a day so he's not going to have time to do a thorough job. I know you have cash flow problems but you're better off paying an RGI to do a proper service than pay for some insurance policy on special offer with terms and conditions and exclusions.
