Try Contract Heating (http://www.contractheating.co.uk/)
I've been with them on a service contract for 21 years and they're great. They also fitted a new condensing boiler for me 10 years ago, and were £3K cheaper than British Gas for the same job. British Gas also wanted to rip up my hall floor and bathroom floor, and leave me to replace it all at my own expense, so the cost would've been even higher. CH devised a different, neat, non-destructive solution to pipe the condensate out of the hall cupboard, across the hall and into the bathroom and under the shower cabinet and bath to plug into the drainage outlets.