Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
First point, did you ever occupy this flat as your sole or main domestic residence?
From the tenor of your question and subject to your response to my enquiry, you will be liable for an element of Capital Gains Tax (CGT) on the sale. For the last 18 months of ownership you are deemed to be in occupation even if this is not the case. Let us assume you sell in 12 months. Your total ownership time is 20 months, knock off the 18 leaves 2. Thus of the gain 400K - 285K = 115K @ 2 / 20 leaves 11.5K exposed to tax. Now deduct your non cumulative Annual Exempt Amount (AEA) of 11.1K and you have 0.4K which would 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. Worst case scenario is a tax bill of a tad over 1.1K.
I do hope that you have found my reply of some assistance.
In that case my answer changes somewhat as Lettings Relief (LR), an additional entitlement, available up to 40K, will knock out the gain altogether. There will be no CGT to pay.
Thank you for your support.