Use a different phone to contact them, they could of blocked your number, Write to them recorded delivery giving them notice before action, add a £25.00 charge for the letter and any further letters.
If you don't get any response then you could issue a summons, but you have to weigh up if it is worth spending more time and money with no guarantee of success. Because you can bet your house if they take it as far as court they will say they weren't satisfied with the job. Which of course before and after pictures prove to be invaluable.
Even if you get a County Court order against them, which could take months, they will still have 28 days to pay, longer if the judge allows them because of their circumstances. Then if they default on the order you will then have to decide to pay further cost for a court bailiff to recover your money. The cost will be added to their debt.
I have gone as far as sending out bailiffs many years ago, and they got my money straight away. Glad I got my money in the end but it was a stressful journey, and one I would not like to go on again.