Thanks for your question - I am Sam and I am one of the Uk tax experts.
First the advsie your accountant gave you regarding a 3 month move into the hossue does appear to be wrong, as you cannot simply wipe out the fact that an occasion arose on this property for capital gain purposes.
However - can I ask
1) Was it the house in Doncaster that was rented out or the one in London, as your opening paragraph is confusing
I get that you lived in Doncaster - then bought a house in London, which you stayed at whilst you worked there Mon to Fri - then I assume you already owned a property in Doncaster - which you then moved out and its that property (in Doncaster) you rented out and sold, on which the capital gain arises.
If this is correct then please advise
1) When you bought the property in Doncaster- how much for and was this in your sole name
2) Did you carry out any capital iomprovements on the property in Doncaster - such as new bathroom or kitchen etc - if so please advsie what improvements and how much you spent (approx)
3) When did you leave the property in Doncaster - to move full time to London - and is this the same date that the rental income began
4) Did you declare all rental income to HMRC
5) What dates did you move back into the property for
6) when did you sell the property - and how much for
I can then advise further
If this is NOT the case then pleae expand on the situation