Proving the share of a tenancy in common

[Tim@highclere-salest]

My father-in-law died earlier this year. His house was owned 'tenancy in common' with one of his daughters. When the property was purchased his daughter contributed approximately 7% of the purchase price. On the deeds, however, there is no record of the proportion of the property she owns, nor on any other legal document. We believe it would be theoretically possible to acquire a bank statement of that time showing the payment of a cheque for that amount.
What does the inland revenue accept in such cases? 7% of the value or do they assume 50% of the value? We need to find the correct answer due to CGT implications.