Hello, I'm Keith and happy to help you with your question. This is not simple, but I will try to summarise your capital Gains tax (CGT) for you. If you have the time, a wet towel for your head and the backs of lots of envelopes you can find the solution from HMRC Help Sheet 281; here is the link:
You will be liable for CGT on 22.5% of the gain made from a market value as at August 2010 and the net selling price obtained by your ex. You have an Annual Exempt Amount (AEA) of 11.1K to offset this gain. Any CGT then due will be taxed at 18% or 28% or a combination of the two rates depending on your income including the gain in the tax year of sale. This capital gain is declared in your annual self assessment tax return. If the sale was in the 15/16 tax year then you would have until 31 January 2017 to settle any CGT due.
Your girl friend would also be liable for CGT on her flat for the proportion of time she rented it (less 18) out against her total ownership time; all calculations being done in months. The last 18 months don't count as she will be deemed to be in occupation even if this is not the case and Private Residence Relief (PRR) is extended. She will get PRR for all the time it was her sole or main domestic residence and PRR relieves CGT at 100%. Furthermore, she will be entitled to Lettings Relief up to 40K instead of an AEA. I suggest that, in fact, she will probably have no CGT to pay at all. I do hope
I have helped set your mind at rest on this matter and cleared the air for you. By the by, you would be entitled to the last 18 month rule too so don't forget to take that into account in your calculations,