UK Property Law
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Thanks for your question.
Yes, there is and it is called a declaration of trust. It is a deed signed by the registered proprietors of the property (presumably both your parents) and it would state that upon sale of the property you would receive an agreed proportion for the proceeds of sale to take account of your help in financing the purchase.
It could be registered against the property as a notice. Both your parent and both of the "we" you refer to would need to sign it as a deed (witnessed).
Any administration of your parent's estate in the event of their passing would have to take account of this deed.
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