Things I've learnt after the first few days.

1. Don't fill the jar; leave a couple of inches: the cabbage rises with the air bubbles and needs pressing down every day for the first few days. I left one inch which was not enough.
2. Put some holes in the cling film! The first morning the cling film had formed a balloon, rising a few inches out of the jar.
3. Keep the weights on the cabbage to keep it down a bit as the gas tries to push it up.
4. The whole house is filled with a lovely fresh cabbage smell.