write it, sign it, have it witnessed by someone reliable. And then keep a copy safe. That said, for the potential value of future claims / liabilities is a simple solicitor’s letter going to be that expensive?
(not like a mate who loaned his ex-GF money to buy a place where they wrote an agreement that should she ever sell then he was entitled to his money back plus a share of any profit, and then didn’t change the locks so she nicked his copy and denied it existed. Had to pay a fortune in court to get an injunction to prevent her selling and disappearing with the proceeds while the case was heard and then about 6 months of solicitor and barrister time to fight the case. In the end the costs were almost entirely swallowed up by his share of the profit and his initial loan back, but in turn she was bankrupted because of her greed and dishonesty. Vindictive? Possibly but crime should not pay, and there was no way he was going to let her walk away with quarter of a million to move in with her new boyfriend)