I found 2 caterpillars on one of the almost leafless decimated purple sprouting today so I definitely know the culprits now. I have run out of nails to attach the new butterfly netting to so will have to wait a couple more days until I am at the shops. I may even buy some more plants then, just in case my newly sown ones don't come to anything.
I am pleased it wasn't slugs causing the damage as it shows my home made traps are working well. I'm not catching many in the new raised bed that has the broccoli but then it was filled with new compost and well rotted manure. The traps in my flower beds show a different story and they are always full of large drunken slugs. I am using small bottles that have previously had sparkling water in them. I cut them where they start to narrow, invert the neck, tape the two bits together and put in some beer. I got the method from here:
https://www.youtube.com/watch?v=cqs4YHUr7Kg The You Tube lady lays her bottles down, but I bury my bottles upright in the soil with about 3-4 inches showing above ground level. I then put a few stones around mine so the little ones can climb in too.