Hi Paul Haynes,
If the disposal of shares under S104, incurs a loss, you would either declare the loss in writing to HMRC, along with supporting cacluations / evidence in cases where a tax return is not required or show the loss in your Self Assessment Tax return and include supporting calulations / evidence.
In some instances there may be a negligible value arising.
Have a look at helpsheet HS283 for guidance:
Negligible value claims and Income Tax losses on disposal of shares (Self Assessment helpsheet HS286)
Thank you.