It's an Aphid of some sort. How big was it? What sort of plant was it on? Could it be a Black Bean Aphid?
John
If you need ID on bugs/beetles etc in Britain then take a look at:-
http://www.eakringbirds.com/eakringb...ctnews2019.htm
A very helpful site run by a knowledgeable couple, who will give good advice.
Certainly worth knowing. Send them a picture if you cant identify it yourself and they will try and point you in the right direction, thats if you cant find it on their extensive website itself.
Thanks Ian