Most of the responses above, whether a guess or not, have this correct.
The gain only applies to the asset (flat) in the tax year you sell it, joint owners divide the net gain made in half to calculate their personal gain, then deduct the annual allowance of £11k, to find the residual sum on which the tax is paid.
Any positive figure residing after deducting the eleven k is subject to the tax at a flat rate of 18% if you are a standard rate taxpayer or 28% if you are a higher rate taxpayer.
The only reason to employ an accountant might be to ensure you take account of all the costs that you have incurred in improving the property over the years of ownership plus all your selling expenses including estate agent and conveyancer.
For many people DIY should be possible.