the contract you have could have been drawn up at anytime though – so id likely treat them as having the £12k and ask explain in order to be satisfied it is a loan repayment and not just a gift to the son, you need them to provide bank statements/other evidence of the showing the loan funds being paid into their account/paid to them.
Id someone said “I’ve paid off my credit cards/ mortgage/ etc” , we would ask for confirmation of that before agreeing it wasn’t deprivation.
