[Sheena]

..on a sage plant.

http://www.rhs.org.uk/research/projects/rosemary_beetle.asp

I thought it might be a Colorado beetle but thankfully not.
 I don't know how much of pest this one is, any ideas?

[Trillium]

Good grief, the UK sure has some unsual pests. It sounds, from the link, that it's much like our Colorado potato beetle, which we organically control with lots of hand picking (and squishing) and dosing with  soapy water with garlic juice since sage and rosemary are edible. The soap stuff will easily rinse off before eating. However, you have to be diligent about finding the egg clusters as prevention is the largest part of the cure.  And there's a chance that a neighbouring host plant might be hiding the eggs. Other than that, there's no safe 'cure' except to not grow sage or whatever