Not sure about the catterpilla but the plant is an Alstroemaria....you might have to get some kind of systemic spray or physically remove and destroy them

it looks a lot like a cabbage white caterpillar to me.

Could be a small cabbage white. Large whites are totally different

is it on a solomon's seal? if so is it one of the sawfly grubs? they skeletonise leaves really rapidly. They don't appear to have the typical anatomy of a true caterpillar.

I've now Googled Sawfly Grubs and it's definitely them, so thanks everyone. I've heard of these little blighters before, but never seen them. Thanks again xx

SBG; No it is the Alstroemeria..or Peruvian Lily. It's a beautiful plant but these little beggars are spoiling it somewhat.

For future interest Large white butterflies and small white butterflies have very different caterpillers, one is green and the other is striped, not sure which is which though, I grow nasturtiums for them to keep them off my crops.

Thanks for the tip Patfran :)