If you have to pay your ex or not would depend on the original contract signed by the two of you I think. But since your situation is complicated and you have many queries, it would be a good idea to visit a conveyancing solicitor with the original contract; and take their advice; they will be in the best position to answer you. There are many good conveyancing firms around; I have used Convex when buying my house. They have a lot of experience, you could try them out. Bye and all the best.