We are looking at boilers right now as ours is on its way out. We are covered by BG and the cover has been worth its weight in gold to use, so I don't begrudge the £23 a month we pay. Each time we have called them they have come out the next day and fixed it, but the parts are no longer available.
We will be getting a local (recommended) plumber to install it and I wil let you know how much he is going to charge us when he rings back.
I don't like combi boilers so we are getting a heat only one which will have a 5 year guarantee so we shan't renew the BG contract until that runs out.