Your mother was entitled to £125,000 when your father died intestate. She was also entitled to the income from half the residue. If there was no other asset other than the house, then she had a life interest in half of £80,000 which wasn't an income producing asset as it was tied up in the house. You were entitled to £40,000. So your immediate interest on your father's death was worth £40,000, the CGT base cost of your share.
Where an individual becomes entitled to a life interest in an asset, that asset becomes part of their estate when they die. Your mother appears to have had a life interest in £40,000 (£80,000 x 50%). When she died the property was worth £150,000. She owned 61% outright (£91,500 plus a life interest in 19.5% (£29,250). You owned the other 19.5% as of right under the intestacy rules. Therefore, your mother's share was £120,750 (£91,500 + £29,250) and that becomes the CGT cost of the 80.5% you inherited on your mother's death. Your total base cost for CGT purposes was £160,750 (£120,750 + £40,000).
You sold the property for £225,000 in March 2015 so you made a gain of £64,250. The first £11,000 of the gain will be exempt from CGT so the net taxable gain is £53,250. If any capital improvements were made to the property before its sale, you can add the cost of those to the base cost of £160,750. You can also claim the selling costs (legal fees, selling agent fees etc).
I'd run this by your solicitor to find out what they think. In effect, your mother was the life tenant of an interest in possession trust which ended on her death.