S.279A TCGA 1992 election

[Brightonian]

I am trying to compose an election and am struggling to understand the legislation. My client sold shares in a company in 2016/17 and, as part of the deal, received deferred cash consideration - both ascertainable and unascertainable. This was all taxed in 2016/17 in order to obtain the ER 10% rate. The client has now received all that will be received in respect of the deal. The last payment was made in January and the total received is short of the amount taxed in 2016/17. I understand that I can elect to carry the loss back to 2016/17 but have a few problems.
I am assuming that the shortfall is entirely unascertainable consideration - I hope that this is the correct approach. I am not very familiar with corporate terminology but assume that the ascertainable consideration will have been paid first. It may be difficult to obtain any information from the new owners of the company.
Is my client 'within the scope of CGT' for 2019/20? He left the UK in July 2019 and will qualify for split year treatment. He made some capital gains before he left - approximately £2,000, so within the annual exemption, but I believe that this is not relevant.
In a normal CGT computation, losses of the year would first be set against gains of the same year. Does this mean that the loss on the earn out which I want to relate back to 2016/17 has first to be reduced by the £2,000 gains and only the balance claimed or do special provisions apply?
I should be grateful for any help members of the forum can offer.

[Brightonian]

It has been suggested to me that a simple amendment of the 2016/17 tax return might be possible under the overpayment relief rules. Does anyone agree?