Simon, at this point I would take steps to try and obtain a full refund. The business has acknowleged that they have made
a mistake which has resulted in poor workmanship and have tried unsuccesfully to redress that to your satisfaction. You have
now lost all confidence in their abilities to do the work. Obtaining a refund may include using the Small Claims Court, but at this
stage will involve you writing a letter following the details and steps outlined in the CA link below to the business owner (sent
by recorded delivery).
https://www.citizensadvice.org.uk/consumer/going-to-court/going-to-court/taking-court-action/step-one-write-a-letter-before-action/
Keep copies of all email records and photos of the work done. Keep all communications polite but firm. Don't post derogatory comments
on their website or FB pages. It may be possible in the first instance to obtain a refund by threatening court action in your letter and is
worth a try at this stage.