caterpillers on my cabbages ? any ideas?

squeaky

It's a fungus that probably wandered in on the breeze from a neighbouring rose somewhere. Commercial fungicides will shift it, though if the infestation isn't too bad you can cut off the affected leaves and put them in the rubbish or burn them. Don't put them on the compost heap! Disinfect whatever shears or cutters you use.

Debt_Free_Chick

If you want to garden with minimum use of chemicals, you really have to think about using washing up liquid - it's a complete chemical concoction :eek: It might be OK to eat food off a plate wahed with Fairy (or whatever), but do you really want to eat food washed with Fairy?

You can buy organic insecticidal soaps. They either contain rotodene, which is a plant extract, or fatty acids derived from some other natural souce.

There's one here and some advice from the HDRA here.