Put simply, if you are using CFDs to hedge then you can include it as capital gains or losses to offset capital gains or losses in your underlying assets.
If you are using it as a one way position bet on which way a share price is likely to move, you can include any gains or losses in the business section (ie you are in the business of trading cfds)
It is somewhat easier to just account for the movement in cfds as a business income or loss as you do not need to really worry about the treatment of margins. However if you have alot of capital losses then it might be beneficial to treat gains as capital gains (assuming you own the underlying shares or similar shares somewhat).
