Selling a joint property

Hello helpful people.. Looking for a bit of advice if possible.. Recently sold a flat that me and the missus used to live in and then moved out and bought a bigger house where we still live.. Back story is.. The wife bought the flat in 2005 by herself and rented half the flat out to a lodger until 2007 when I moved in and paid rent. Then in 2008 i bought into the flat (took on half the mortgage) and then we got married and then lived there til 2012 when we moved out and elsewhere and it has been rented since then.. Now it is sold i need to work out the capital gains tax payable.. For her it is fairly simple.. For me, less so.. Will the value of the property for me be the price she paid in 2005 split 50/50 or will it be the price (split 50/50) that we valued it at when I bought in 3 years later.. Also will she be able to claim lettings relief between when she bought it and when I took over half the mortgage. thanks all.

Hi

Yours will be when you took on the mortgage and the your share of the value at that time.
For lettings relief, please refer to CG64710 - Private residence relief: letting: relief for letting of residential accommodation

Thank you