Reading between the lines, this does not sound like a proper transaction at all. What % of the property is in her ownership? Are you not planning to use a solicitor in this process?
It seems to me that the proof of payment thing is irrelevant. You're not buying TV. Treat the transaction with the respect it deserves. Engage a solicitor. The exchange of contracts with a specific stated value is your 'proof', not a stub in your cheque book showing that you actually paid.
Maybe I'm over-reacting. Also, this borrowing from a friend piece is disturbing, but I won't go down there.