You could always put it in writing that they are on notice to complete the job as per the original agreement within 14 days, otherwise you will get it redone and any additional reasonable costs as a result of that will be down at their expense.
As long as you give the required notice and use registered post or some other way to prove they got it, then you're free to raise a small claim for the difference. If they know that, then that might clarify their mind to finish off.
If you don't want the stress as per doggit suggestion just find another electrician and hopefully one will take it on at your own cost