It turns out that they are Rosemary leaf beetles , Chrysolina americana, and they are native to Southern Europe despite the 'americana' name.
I loved the metallic green and purply stripes but they were quite difficult to photograph because they were so shiny. The last time I had such problems with strange reflections was when taking photos of a highly polished vintage car at a wedding.
It was very hard not to be reflected in the metallic gloss- in fact, the dark blob on this beetle's back, is my reflection. I suppose it makes the camouflage very effective.
The beetles are very beautiful but can be annoying for gardeners as they can cause some damage to herbs such as rosemary, lavender, thyme and sage plants. Relatively recently, they have become established in the UK