We are currently doing a lot of plumbing works in our house including installing a new storage boiler, removing the old, moving it's location, installing new rads etc.
We're using Maintracts Services
http://www.maintracts.co.uk/ as they're very local to us in SW17 and get great checkatrade feedback. Very happy with their communication, availability, advice and work so far, so would totally recommend for major heating works.
For instance, I didn't even know there was such a thing as a storage boiler - a sort of halfway house between a combi and using a system boiler with a storage tank. Malcolm pointed this solution out for my property and it's what we've gone for to run a small 4-bed, 2-bath property.
SFK