Second about the wasps - don't need to net brassicas, the wasps will be around feasting on the eggs.
If you can attract frogs into your garden ie anywhere cool and damp you'll find them romping around expeditiously at nightfall to deal with any mollusc problem. If I do find them I'll often get them onto a sheet of paper and put a ring of salt around them - in fact it's a bit of a scientific experiment I've had going for a few years now, I write up which way they go trying to escape, how long before they terminally insalinate themselves, how long writhing around etc, and one day I hope to publish the results - you don't have to be a genuine scientist to be allowed to state the bleeding obvious, do you? - "Landbound molluscs display striking similarities to hominids when trying to escape from tricky situations". And if you ever thought slugs are slow, try putting the newspaper on top of a hot stove - you'll find they are capable of a surprising turn of speed.
