Was there any written agreement at the time about what would happen about paying back the money? Did the solicitors on the purchase query it in any way (money from a third party if you sent it in to the solicitors direct) and get any agreement down? Could be worth asking as solicitors are regulated as to due diligence in relation to the source of third party funds (anti money laundering etc...).
If not, it could be difficult getting the money back without his consent. If the property has sold then you cannot register a charge against it (registering a charge is not simple in itself with no consent or agreement or a Court Order) once it has changed names as the debt has nothing to do with the new owners.
The property now belongs to someone else and if their legal advisors did they job properly there will be a Land Registry priority on the property meaning that their purchase and any mortgage has priority for registration up to a certain time over any other applications against the property.
Your best bet at this stage is to try and get his consent. If not, would you have sufficient grounds to get a court order eg a written agreement or at least some kind of formal evidence of an agreement - verbal is hard to prove! Could you prove it was a loan rather than a gift - and to them both rather than just your daughter - and any terms of repayment agreed eg, if they split and the house was sold you would get your money back from them half each? Similarly, did they have any agreement as to what would happen?