Looking though my insect guide this looks like it could be an aphid. My guide shows Tetraneura ulmi being the size you state and being green (without the brown marks) and a similar shape. Images on the Internet look nothing like that in my guide though! There are aparrently 30 species within the Tetraneura so maybe this is one of them.
I can't tell you what species this is, but it IS a Sheildbug nymph. They go through about 5 instars to adulthood. and sometimes wind up in places they shouldn't be, like your house. It does look like an early instar, which is why I couldn't say which one. but a good site for this family of true bugs in the UK is at British Bugs.
Hope this helps
Good shout. My insect book infuriatingly says nothing about instars and only shows adults. I looked on the internet and found the following
which looks very similar to the bug in the question.