Hi Hamish Down,
Your father will not be liable as it is his main residence.
You will be able to claim private residence relief for 1994-2010.
1994 is the start date as this is when you 'acquired' the property.
You will need to obtain a valuation from 1994 and this can be done via local estate agents.
You will need three to then take an average.
Alterntively you can ask HMRC to do this:
Post transaction valuation checks for Capital Gains (CG34)
For private resdience relief refer to:
Private Residence Relief (Self Assessment helpsheet HS283)
Thank you.