A client won't pay a 2500 pound invoice — how do I write off the bad debt in my company accounts?

Firstly, I'm sorry you're dealing with this — it's a common headache for small consultancies. Because the invoice is over six months old and you're on standard VAT accounting, you should qualify for VAT bad-debt relief, provided you have actually written the debt off in your VAT account records. You reclaim the VAT element on your next return via Box 4 (and reduce your output VAT in Box 1 if you accounted for it when the invoice was raised). For corporation tax purposes, a debt you have taken all reasonable steps to recover and formally written off is normally an allowable deduction against your profits, so do record it as a bad debt expense rather than leaving the invoice sitting in receivables. Note the distinction: a credit note cancels the original supply itself, whereas a write-off reflects that a genuine supply went unpaid — only the latter supports bad-debt relief. Finally, keep evidence for HMRC: copies of the invoice, your chase emails and call notes, the ledger entry showing the write-off, and any correspondence about the client's insolvency, since HMRC can check that both the six-month and written-off conditions were met.