Hi Mark Arnold,
There is a Capital G Taxains calculator here:
Tax when you sell your home
One of the questions in the calculator will ask if you are claiming Capital Gains Tax losses from tax years before 2023 to 2024 that you have already reported to HMRC. You will need to enter the losses in this box, even if they occured in 2023 to 2024.
When completing the Gapital Gains Tax online form, there will be a section for in-year losses.
The losses would be declared there.
Please note that you have 60 days from the completion date to report and pay any Capital Gains Tax.
Thank you.