However you dress it up, you cannot beat them with a stick until they refund your money and your only remedy is to take them to the Small Claims Court.
I would give them a deadline (in writing) to refund your money by, say, 4 PM on Friday, 28 January 2022 failing which will issue Small Claims Court proceedings without further notice.
The new Moneyclaim beta system is easy to use: https://www.gov.uk/make-money-claim
the new system is much better than the old government Gateway system.
You can prepare the proceedings now, at your leisure, save them, and then just pay the fee and press the button when the deadline in your letter before action, expires.
It's then just a case of following the court process and responding to any emails that you receive.
It’s not expensive.
A word of warning and that is that they may put forward a defence that this was an informal family arrangement never intended to create legal relations. (That is the legal terminology) and the court may decline to intervene just leaving you to fight it out between yourselves.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have