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Hello, this is Jim, a dual-qualified lawyer (UK & Republic of Ireland) and happy to help you today.
You need his agreement to sell, or a court order if he refuses to sell. The court will look at evidence of the agreement - the deed of trust would have been ideal but if there is no record, then you will need to provide evidence of your 90% and the further mortgage payments compared to his. It all depends on the evidence available - no doubt you can evidence higher payments and the burden of proof in a civil court is only "in all likelihood", so if you can show you paid more in all likelihood then the court will agree that you have done so.
If you sell the property then the solicitors can retain the funds until this is resolved. Or if he refuses to agree, you can apply to force a sale or apply to remove him from the deeds (the latter if you want to keep the property for yourself). It is a technical area of law so a law firm such as http://www.blasermills.co.uk would be ideal - they could threaten the ex with costs if he continues to cause you a problem.
I hope this helps? If I have answered the question would you kindly leave me a 5 star rating (at the top of your screen), any follow up questions are welcomed.