This very small green spider, body no more than about 2mm long and maybe not even that, was on a teasel plant - that's the stem of one of the long spikes that surround the teasel flower itself. It appeared to be able to jump but it finally disappeared from view by spinning a length of web which it used to "fly" away on the breeze.
I though it may have been either a crab or green orb-web spider but I've not convinced myself of either. I prefer the green orb-web option but it's not like another one of these I've seen and positively identified.
I've now been given a probable id for this spider - Nigma walckenaeri