Firstly what is wrong with your back boiler? Has it been condemned? If so then to replace a back boiler or install a wall mounted model will cost virtually the same. Assuming you have a fully pumped system already and your hot water cylinder is to current standards, then you are looking to pay out around the £2000 mark. In my experience do not go out and buy a cheap boiler as a replacement - it's false economy and won't last as long for the extra couple of hundred for a decent model with a 5yr guaranteed warranty. Get at least a couple of quotes from reputable gas installers as a site visit will be required.
moggridgeplumbing
March 2011