I rang AgeUK to recommend a plumber in Sheffield to do some plumbing work at my mum's.
They suggested I use the TrustMark website.
http://www.trustmark.org.uk/
Apparently they are government approved (for what that's worth?), no idea what they charge, but I would definitely go by recommendation from a friend first.
Looks like that website could do with some DIY, cos it's down at the moment!