Personally I would get a loan agreement drawn up by a Solicitor. They have standard ones so shouldn't cost much. I made a loan to a mate a few months ago, and the loan agreement went into pages. Belt and braces I imagine. You need to prove it is a loan, otherwise when the beneficiary dies the Revenue could deem it a gift, and it could be part of their estate for Inheritance Tax, as well as being not repayable to you.
I suppose it would also depend on amounts. if you are talking tens of thousands definitely get a solicitor. If you are talking the odd thousand you need to take into account the solicitors fees more.
When I made my loan, the beneficiary paid my solicitors fees.