Hello, my name is Jim and I am a qualified lawyer happy to help you today.
With your mortgage contributions then you can establish ownership with a "beneficial interest". In law that means the court recognises your contributions and makes you a co-owner. Furthremore if you and your partner agreed that the house would also be yours, you can say that there is a "constructive trust", which means there is an express or implied agreement (a "common intention") that the ownership of the property would be shared between you both. From what you say, there is likely to be a finding of a beneficial interest and possibly a constructive trust and that being the case, you can ask the court to make an order that there is such a trust and that you have a share in the property. Once that happens, you can force a sale of the property to release your share.
In the meantime you can apply to the local county court for an Occupation Order which lets you live in the house despite what your partner says or what he wants to do.
Once you have established a beneficial interest with the court you can then apply to force a sale under Section 14 of the Trustees of Lane and Appointment of Trustees Act 1996 and you need to quote Rule 73.10 of the Civil Procedure Rules 1998.
I would recommend a letter or email is sent to your ex to confirm the above and that either they let you stay in the house or they pay you a sum to reflect your contributions to the mortgage. As a priority you should apply for an occupation order.
If you do the latter, whilst it is possible to do it yourself, you may want a law firm to handle the procedure. They can also seek their costs from your ex if you have to involve the court. I can recommend the following law firm to handle this for you: https://www.blasermills.co.uk/contact
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