It was a Hawthorn Shield Bug (Acanthosoma haemorrhoidale).
It was very co-operative about having its photo taken, and afterwards I put it on a cotoneaster bush, as I think it must have dropped out of it while I was pruning.
When I went out again an hour or two later, there was a second Shield Bug sitting close by, which I hadn't noticed before. It must have been attracted by the presence of the first one.
These bugs go through five larval stages called instars.
This morning, when I went to the garden waste bin to add some more prunings, I discovered what I now recognised as an immature Shield Bug inside the bin. (I also made a mental note always to shake any prunings before putting them in the bin in future, so that these small creatures don't get sent away with the waste)
Later I found 2 more smaller ones. This is the biggest and the smallest at 11:50am...
8th October 2009
Today I found this shieldbug crawling on the lid of one of our compost bins. It is a Birch Shield Bug (Elasmostethus interstinctus) and it measured 8.5mm excluding the antennae - slightly smaller than the Hawthorn Shield Bug. Note that the projections on the pronotum (the widest bit just behind the head) are smaller and not tinged red.
16th October 2009
It has been a while since I saw any Hawthorn Shield Bugs, but I discovered this 5th instar today on the cotoneaster bushes.
More information can be found on the British Bugs website.