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.

 Making Sauerkraut

1. Bought two 4.25l jars with 10cm openings, https://smile.amazon.co.uk/gp/product/B08P9VVSDD/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
2. Followed the BBC recipe https://www.bbcgoodfood.com/recipes/simple-sauerkraut but with 4tbsps salt per 2kg which seems to be the consensus across the web.
   
3. The recipe says 15g per 500g later so I should probably have done this!
4. Shredded four white cabbages, which ended up filling up one jar nicely. Worked in the salt in a mixing bowl for 7-8 minutes in three batches.
5. Pounded down with a pounder.
6. Used tinned tomatoes wrapped in cling film as a weight.
7. The liquid formed quickly. 
8. House filled with a lovely cabbage aroma

Now I just have to wait 3 to 6 weeks!