There are two species which are virtually identical- the rutherglen bug (Nysius vinitor) and the grey cluster bug (Nysius clevelandensis).
Both are grey and black in colour, with some brown markings. They are very difficult to tell apart but under magnification the Rugherglen bug is smooth whilst the Grey cluster bug is hairy.
Nymphs are wingless, pear shaped, reddish brown and look a little like an aphid.
Adult size: 5 mm