Thanks to Dr David A Kendall of Nailsea, Bristol. I sent him some pictures of the bug and he came back with this reply:
Immature (nymph) stage of a shield bug or stink bug, but cannot determine species at this stage from photo.
Here is a link to a webpage for you stink bug enthusiasts
http://ipm.ncsu.edu/ag271/soybeans/stink_bugs.html
ipm.ncsu.edu/.../stink_bugs.html