How do i get proof of a business Partnership Agreement for a PPI claim?

Hello, HSBC have upheld my claim and agreed i am entitled to a share of PPI with my ex-husband after lots of to-ing and fro-ing! However, in their letter they have said that if my ex husband does not agree to a 50% share each (which i highly doubt he will) then i'll have to produce evidence of a Partnership Agreement on the joint business that we had. Who do i need to speak too, to get hold of this?

Many thanks