A letter before action (also called a letter before claim) sets out what is owed, why, and the deadline to pay before you go to court. Courts in England and Wales expect one; skipping it can cost you interest or costs even if you win.
If the debtor is an individual or sole trader, the Pre-Action Protocol for Debt Claims applies: you must give 30 days to reply and enclose the protocol's Information Sheet and Reply Form. For companies, 14 days is generally reasonable.
Example
"Unless the sum of £2,350 is paid by 14 November, I intend to issue a claim in the County Court without further notice."