There's a good website here but you might be surprised how many different species of "green-fly" there are!  You'll probably need to know what plant they were on to have any chance of identifying them. 

Hi, I've attatched a picture of the plant, which I'm also hopng fo an ID on.

The plant is Wood Avens.

https://www.naturespot.org.uk/species/wood-avens

I've rang the person whose garden I found the greenfly in and apparently there are rose bushes quite close by if this helps.

Wood avens (Geum urbanum) is also known as Herb Bennet.  There is a Herb Bennet aphid (here and here) that is bluish-green and looks a good match for yours but I'm not expert enough to make a positive identification.