Borrowing from family

I''m borrowing money from a family member to pay off my ex. The agreement is that they then ''own'' X% of the house, to be paid back when it''s sold, no deadline. The house is mortgaged, but there is adequate equity to cover this.
We want to put this in writing between us, would I need to see a solicitor to draw this up or could we just put it on paper ourselves?
We have the Nisi, are doing the Consent Order at the moment and Absolute is the end of this month.

Not sure if you can do it yourself but I think it''s called a declaration of trust - if set up through a solicitor I know fees vary but I had one done it cost approx £100 - in the scheme of things I guess it was worthwhile as I''m not sure if you draw a document up amongst yourselves how this would hold up legally??? I''m not an expert and this information is only given on my experience - I''m sure someone will correct me if I''m wrong!