Here’s the things I’ve used comfrey for to date:
1. Compost piles – comfrey is a really great accelerator in compost bins.
2. Hilling up potatoes – I routinely put plenty of comfrey leaves (I don’t use the stalks) down when I am hilling up – I also use spent grass cuttings.
3. I use the liquid comfrey made like Charles describes when runner beans are flowering – I’ve seen marked acceleration in bean development, particularly when used on fruit days.
4. Tomatoes – I’ve not done any formal trials vs Tomorite yet, but it’s certainly worthwhile.
Nettles I have less experience with, but one tends to think it should work best where a nitrogen fix is required: brassicas, vegetative phase of peas and beans, chard?