And if so, can anyone point me to relevant documentation
It’s explained at the link above.
Gains and losses from CFD trading are usually charged under the capital gains regime. So you are currently allowed £12,000 gains per year tax free, and above that taxed at 20% of gains.
Note that dividends received on CFDs are to be declared and taxed as capital gains, not as ordinary dividends. Similarly, the cost of overnight interest is also part of the captial gains calculation.
Thank you for the answer!