Was your name put on the deeds when it was put on the mortgage in the 1990s?
If you became a 50% owner in the 1990s and then the full owner in 2008, then the cost of the property for CGT purposes will be the sum of 50% of the value when you acquired it in the 1990s and the value of the 50% you acquired in 2008.
Assuming your mother continued to live in the property, the gift with reservation of benefit rules may apply. See the notes here and here.
If you have lived in the property during your ownership of it, then you will qualify for main residence relief for that period. See HS283 for information on the main residence and CGT.
I hope this helps but let me know if you have any further questions.