Araniella sp.....

There are six species of these green Araniella species found in the UK and while at least two of them are considered rare, they are all indistinguishable from one another in the field without use of field microscope!  So, although this is probably one of the common ones, such as cucurbitina or opisthographa, I just can't be sure...it is a male though:

Araniella sp: R. Harris