A client never paid a £4,000 invoice but I already paid the VAT to HMRC — can I get it back?

Yes — this is exactly what VAT bad debt relief exists for. If you've accounted for VAT on an invoice and the customer never pays, HMRC lets you reclaim that VAT, provided you meet the conditions in VAT Notice 700/18. The conditions are straightforward. You must already have accounted for the VAT on the supply and paid it — which you've done. You must write off the debt in your day-to-day VAT accounts, transferring it to a separate bad debt account. And the debt must have remained unpaid for at least 6 months from the later of the date payment was due and the date of supply. Since your invoice is over a year old, you've comfortably passed that test. You must also claim within 4 years and 6 months of that later date, so don't sit on it indefinitely. Claim by including the VAT in box 4 of your VAT return for the period in which you become entitled to relief, and keep a bad debt account recording the invoice number, customer, dates and amounts claimed. One final watch-out: if the customer ever does pay after you've claimed, you must repay the relief to HMRC through box 1 of your return.