With no trust declaration in place, then you don't have anything legally to protect any additional payments that you have made towards the mortgage.
However, it may be possible to imply or declare a trust after the event, if needed.
It should hopefully not come to this.
As to what is a reasonable buyout, start with the equity at 50% each, then make any deductions you feel are fair in the circumstances, if you have contributed more over time. Be realistic, also note that if your co-owner contributed towards or paid for other joint expenses outside of the mortgage then this may counterbalance any argument you make for a larger than 50% share. Of course if one person worked/ contributed less to enable them to stay at home looking after children, then penalising them for this in terms of a lower equity share would be unfair.
You are at the negotiation stage, so try to be reasonable and agree a sum that allows you both to proceed with your lives moving forward.
I hope that this helps.
Do remember that I can only give you my opinion.
Another lawyer may have a different opinion. If there was a black-and-white answer to every legal problem there would be no need for anything to ever proceed to court!
I am glad to help.
Hopefully, I have answered your query in a way that is simple and easy to understand.
I would be more than happy to clarify anything else for you. In the meantime, thank you once again for using our services.
I am happy to answer any specific points arising from this.
Please be aware that my answer is based strictly upon the information you have given me.
If you still need any points clarifying, I will be happy to reply because the thread does not close. In fact, it remains open indefinitely.
I am always happy to answer any further questions you have on any new thread in which case, please start your question with, “ For PLCLEGAL only”.
That only applies to new threads, not this one. You have me exclusively on this one.
Take care and stay safe.