Been thinking about what Steve's doorstepping cowboy builder would need to do to commit a criminal offence. It isn't that easy to prosecute someone for this sort of thing. As a consumer in a free market I am free to strike any bargain I wish. If I choose to pay someone several thousand pounds to do work on my roof that doesn't need doing, I am free to do that. Someone accepting my offer to pay them for that service is not necessarily a criminal, although they might well be be fleecing me.
So, had Steve been better prepared, and strung his cowboy builder along for a little longer, what might our friend have done which would be a definite criminal offence? Here are some suggestions:
1. Steve could have agreed to have the roof fixed, but asked the guy to come back in a few days time to do the work. He has then entered into a (verbal) contract in response to an unsolicited approach at his home, so Doorstep Selling Regulations kick in. Our cowboy builder needs to inform Steve that he has a statutory seven day cooling off period. Failure to mention this means a visit to Magistrate's Court and a maximum £2.5k fine.
2. Get him to fix the gas cooker. Gas Safety Regulations, certainly a fine, a lot more if he breaks it/leaves it unsafe and endangers the house.
3. Get him to do add a couple of new sockets in the kitchen. Part P of the Building Regulations prohibits this unless our cowboy builder is a "competent" electrician (a member of NICEIC or similar body), which is unlikely. (Although strictly speaking, the detail about whether you should be adding new sockets in kitchens is in the accompanying guidance -- the Approved Document -- to the building regulations and doesn't actually have the force of law. If our cowboy managed to safely install these sockets, it is unlikely (I think) that he could be prosecuted.)
4. Trade Descriptions Act. He's probably breaching this just by recklessly claiming the roof is in urgent need of repair. And what was that bit of paper he was waving, which he seemed to be saying showed he was "a genuine builder"? If it was false or misleading, then that too would get him nabbed under the Trade Descriptions Act.
That's the best I can do, which is probably enough to get him a few healthy fines, but unlikely to keep him off the streets for long.