What is eating my overwintering Chinese cabbage

My Chinese cabbage, growing under plastic cloches, looked fine until today, except for some irregular holes in the outer leaves. Now, as the hearts begin to form, they are being destroyed. There is also loads of frass in what remains of the heart. I managed to find a few small black slugs and several grey/green caterpillars, which have all been despatched, but I don't know if they are responsible.

Can anyone suggest the most likely culprit and any remedy (non-organic will be OK)