You should refer to the HMRC helpsheet HS286 here, the notes here and the HMRC negligible value agreements list here. Whether you can make a claim for a capital loss depends on when that loss was incurred.
If you still own shares that have become of negligible value and the company has not been dissolved and HMRC have not already given a date for the negligible value status to be effective from, then you can make a claim for negligible value status to apply for the tax year in which you make the claim or either of the two previous tax years so long as the shares were of negligible value in whichever of those two tax years you make the claim for. See pages 1 and 2 of HS286.
If the company has been dissolved, then you no longer own the shares and the date of dissolution of the company is the effective capital loss date unless you have made a claim before that date.
In some circumstances, the capital losses can be offset against income or gains. HS286 has more information on this.
For losses incurred from 1996/97 to 2010/11 inclusive, you have four years from the end of the tax year for which you wish to claim the loss if you complete or completed self-assessment tax returns for those years in which the losses were incurred. If you don't complete or didn't complete self-assessment tax returns for the years in which the losses were incurred, you have five years from the 31 January following the end of the relevant tax year to make a claim by letter to HMRC.
For losses incurred in 2011/12 et seq, you have four years from the end of the tax year in which the loss was incurred.
If losses are not claimed within the time limits you lose the right to use them I'm afraid.
I hope this helps but let me know if you have any further questions.