This is one of several very similar Dolerus species, the males being all black, but the females have an orange/red pronotum. With all Dolerus species detailed examination is recommended in order to identify them to species level.

males, females

Wet grassland and marshy areas.

April and May.

Widespread in Britain but infrequent or not well recorded.

Rare or under recorded in Leicestershire and Rutland. 

https://www.sawflies.org.uk/dolerus-poodolerus-haematodes/