Right I closhed! I closhed everything remotely yummy looking with bog old fleece tents and something has got in and massacred most of the broccolli and has spread to the brussels and savoy cabbage

How very dare they!!!!! Last year we got 3 weeny half eaten heads of broccolli out of 15 plants and this year seems to be going the same way. so please help what are the buggers and how can I get rid of them. At the mo I am picking them off one by one and squishing them

which is gross but if I can spray them with something organic which will anihilate the little green inch worms!!!!!
So the offending critters are - light green with a lighter green band towards the bottom of them. I can't find any eggs but I have found loads of weeny ones attached to the leaf by a sort of gauze like thin gossimer sheets. they get to about an inch long and seem to have 2 feet at the front and 3 at the back and move like inch worm from disney movies.
Please help me as I want at least some brocc and cabbage this year
Thanks in advance!!