I know a small contractor (not in the domestic world) and he would get some sort of court order freezing the debtors bank accounts. I think, all he had to do was provide evidence of the debt and that was that - he always got his money.
With Limited Co's that's the absolute best way to get debts paid. Wife is an ACCA accountant as has to recommend such action 10+ times a year, if the business is trading (and wants to keep doing so) the debt will get paid.
Back to the OP..........
Legally you would be done for breaking and entering, malicious damage of property (plaster, sockets, walls) since to reclaim materials you would need access and would need to destroy 3rd party works (decor, plaster, paint)- So you can't do that !
You may well find that the 2nd fix sparks was knocked just like you !
You must establish who the person was, where he lives and where his works would be useful.
I'd assume that on the email he made instructions and agreed the work project start, content, etc
I'd assume a verbal contract (if all else isn't correct) was witnessed between you and the guy via your work partner / mate.
You then get a choice, ask a debt recovery firm to have a go- which keeps your hands clean, although adding to the costs.
You go and have a go- which could lead to complaints of intimidation, lies about assault etc
You go to the small claims court.
Personally I'd get his full name, check to poll register, check the land registry, find where he lives and works.
I'd then get a couple of debt recovery folk to sort it out. As a market trader he is likely to be a bit dodgy, ***** (without giving offence to gypsies) and so 2 big ex bouncers doing your dirty work would be best.