Grey colour bug, short wing-like structures not covering abdomen.
No interactions present.
The Ant Damsel bug is also very similar (Himacerus mirmicoides), but I think the combination of habitat and antennae length make the above species more likely. There are good pictures of both species at http://www.britishbugs.org.uk/gallery/heteroptera/Nabidae/nabidae.html
Fiddly one this - I looked at Himacerus for the same reasons as Rob, but this should have definite spots along the sides of the abdomen. Nabis usually have long wings, but in short-winged forms, the rear edge of the pronotum is about 2.5 (and definitely more than 2) times as long as the front edge.
This is certainly a Himacerus female and as Rob says, the length of the wings and antennae plus the arboreal habitat all point to H. apterus.
The spots on the sides of the abdomen in this genus are not always very obvious and seem to be almost absent in this specimen
National Recorder for Terrestrial Heteroptera (shieldbugs & allies)
Ta for info - having been off to look at the BritishBugs website, I'd have to agree,
Lat/Lng: 52.0, -2.7
OS grid ref: SO5537