Post-transaction valuation check
It is possible to ask HMRC to check valuations (for capital gains tax) after a disposal has been made but before you submit the relevant tax return. This service is available to trustees, companies and individuals.
This service can also be used for negligible value claims where one has a chargeable asset which is worth significantly less than it cost (something more than 95% less is a benchmark) and you would to treat the asset as disposed of and reacquired at its negligible value.
The form for requesting post transaction checks is a CG34.
HMRC have recently announced that the time you should allow them to check a valuation is now three months (it was two months), so do not leave it until the last moment.
If agreement is reached with HMRC then there should be no enquiry into the valuation when it is used to complete the self-assessment tax return.