Delia-esque Slow Cooked Root Vegetable Soup

I was meant to be making this soup today but at some point between reading the recipe and chopping the vegetables I managed to convince myself that I needed carrot, parsnip, celeriac and swede rather than carrot, leek, celeriac and swede. Hopefully it will still be nice (and I needed to do something with the parsnip, so it may as well go in there), and it's an excuse to make the soup again using the proper ingredients!