Bring the stock to the boil in a large pan, and add the herbs and Marmite. In the meantime, cut the swede into 1cm pieces. Add the brown rice and swede to the pan and simmer for 10 minutes.
Wash and cut the carrots and leeks into 1cm pieces, add to the pan and simmer for a further 10 minutes.
Mix the cornflour with a couple of tablespoons of cold water, and add to the pan. Add the peas and sweetcorn, and simmer for a further 5 mins. Taste and adjust the seasoning if necessary.
Notes
You can add other veggies to the soup depending on what you have available. Add firmer vegetables such as swede and potatoes first, and softer leafy veg at the end.
If you don't have (or like) Marmite you could also use soy sauce or nutritional yeast.
You can use pasta rather than brown rice if you prefer. It will take less time to cook so add it at the same time as the carrots.