1. Your contract with your ex should have been written in such a way that it was a restriction on the land title (i.e. it should have been registered at trhe Land Registry so that he could not sell the house without paying you the money). If you had solicitor's advice when the contract was drawn up, & it was prepared by the solicitor, it might be (depending on the details if the situation & what instructions the solicitor was given) that you have a claim against the solicitor for negligence.
2. I'm not sure whether the criminal prosecution is in connection with the contract you have with your ex. From what you say I don't think it is, but if it is, you should make a Court application for compensation. Your solicitor should be able to advise you on this.
3. I'm not sure why you think his assets will be frozen or sold. If they are sold this presumably will be to pay the bank which he defrauded. It might be that the sale will realise sufficient to pay them off & leave something over, which you could claim. Again, you need a solicitor to advise you on this.