Length ~5mm. Females have deep blood red coloration and red femorae. Males are dark or reddish and indistinguishable from P. betuleti, with which this species was confused until very recently. 

Psallus betuleti

female male

Dissection of male specimen required to confirm ID, please supply habitus photos plus images of aedeagus. Photos of females should clearly show red femorae

On Birch.

Summer.

Uncommon in Britain.

Rare, though possibly under recorded in Leicestershire and Rutland. 

https://www.britishbugs.org.uk/heteroptera/Miridae/Psallus_montanus.html