Receiving loan repayment from friend

Years ago I loaned a substantial sum (£8000) to a friend, and he will now repay it into my bank account. If HMRC decides to investigate and starts asking questions about the money received, how can I prove this was a loan that is now being repaid? The loan was made at zero interest, and I will not make a profit from it.

Hi,

You will need to have written agreement signed by both parties to show this.

Thank you.