What is eating my cauliflower plants, and more importantly how can I
stop it?

I have Summer cauliflowers and summer cabbages planted together under
netting. The cabbages are untouched but the caulis have been
devastated. The hearts of the plants have been eaten and some of the
leaves chewed off at the stalk and are lying on the ground.

Any ideas on the perpetrator, or how to find out what it is would be
appreciated. I think they are too far gone to save, but I don't want my
Winter caulis to go the same way.

TIA
John