It's like a giant aphid , about 8mm, but I think it's the nymph stage of a bug?
No interactions present.
Looking at http://www.britishbugs.org.uk/heteroptera/Pentatomidae/palomena_prasina.... it seems to match 4th instar, possibly the next stage though. I should have noticed the detailed markings, I'll know next time.
Paul Seligman. Flickr: http://www.flickr.com/photos/paul_seligman_cardiff/collections/721576087...
Lat/Lng: 51.48561, -3.26789
OS grid ref: ST120770
Map is precisely marked