Spring cabbage

No. Deffo eggs. Individual eggs. Not clusters. Mer am at a loss. Must be something small to get through the netting.

It's about 1/4" squares. I'm tipping its a moth. I've been watching and no butterflies have got past it. When I've found them, the caterpillars are only tiny little lime green things.

If the growing point it still there, then they should recover if you clean them off and cover with fine netting and put down slugbait
cos if the caterpillars don't get them - then slugs will 

I grow greyhounds, and it's too late to sow anew. I wouldn't bother.
However, they are very resilient and are quite able to come back strongly after a bird/slug/butterfly attack. Sluice off the muck which will also give the plants a good water, pick off any remaining caterpillars and slugs that remain. Then a few more pellets, and re-net. There's a good 5 weeks of decent growing time and depending when you planted, you might get some decent hearts... greyhounds don't take an age like savoys, they do it quite quickly.