Well it is a nymph (wings not fully developed), so that complicates identification.
I am fairly sure it is Pentatoma rufipes ('forest bug' or 'red-legged shieldbug').
- http://www.britishbugs.org.uk/systematic_het.html (includes list of Pentatomidae in Britain)
I'd like to confirm Mike's I'D; - a nymph (also called instar stage) of P. rufipes.
This one is in its final (5th) stage before it becomes an adult - a indicated by the well developed wing-cases.
Unlike most shieldbugs, these overwinter in their early instar (about the size of a large pin-head - but far more cute) stages - and can be found wantering around on any mild day in winter.
These were taken on 12 December 2011: http://flic.kr/p/aUKSaF - scroll down to get an idea of their size at that age.
Well, over a couple of years I really got to know Forest bug instars - for some strange reason they inhabited the walls of my local brick built bus shelter. It was a quite remarkable refuge for small beasties - one of the few sightings of Eremocoris fenestratus (until recentle thought extinct in UK) occured there.
Sadly the bus shelter was removed (to be replaced with a more modern one) in March this year.
All of my Bus Shelter denizens are remembered here: http://www.flickr.com/photos/fence_s13/sets/72157628021851041/