My Friend The Caterpillar

There have been very few butterflies about the garden this year and so by extension, very few caterpillars. In fact this is one of only two found lurking on the lid of our log box this morning. All the butterflies seen so far have been Cabbage Whites so I naturally assumed this was one of their offspring. Having looked it up on the Internet though, I am not so sure. The examples coming up were not exactly this colour, nor as smooth skinned. Anyone know what species this is?