Hi
This sort of issue has been in the news a lot recently. There is apparently not much you can do, without incurring additional costs that you might not get back. If you entered the property even with their permission, they could make unfounded accusations against you and this could make matters worse.
Write to the person by recorded delivery and threaten legal proceedings if they do not pay up within 7 days. If they don't pay you could think about taking this matter to court and getting a judgement against them. This will cost you and you may not even then receive the money you are due, but you may get some satisfaction.
The other unorthodox way, could be to pay an unemployed friend with bottle, to stand outside their house with a banner, saying this person had electrical work done for £x and did not pay their bill. This might embarass them enought to pay up and as far as I know is totally legal, as long as they don't block a public highway or cause a public nuisance.
Huckster