"Mmmmmmm comfrey" to quote Allan Shepherd from his "The Little Book of Compost"(1)
"... picture me at my desk with a glazed look in my eye and a cascade of dribble sliding over my chin towards QWERTY. Comfrey to Allan the composter is donut with double caramel topping to Homer the fridge raider. This is a mouth-watering treat of a plant. It's easy to grow, perfect to compost and great to turn into a liquid feed"
The beauty of this - let's be honest - rather ugly plant is that it sucks up nutrients hidden deep in the ground and accumulates them in their leaves. These can be composted or turned into a liquid feed for other plants in the garden. There is more information on comfrey in Allan Shepherd's book and at Comfrey http://www.allotment.org.uk/grow-your-own/comfrey. Allotment.org.uk describes the comfrey in water method for making liquid feed, which has the disadvantage of smelling "like an open sewer when finished". Allan Shepherd favours the less smelly, dry, holed bucket approach one version of which is shown in this YouTube video at https://www.youtube.com/watch?v=iRbHqz1m7kk.
A big disadvantage of comfrey as a plant in the garden is that the original, wild version is invasive. If you don't want to spend time pulling up plants that keep popping up where you don't want them you need a variety called Bocking 14. I discovered that one of my permaculture friends on Facebook had some in her garden but if you are not so lucky the Organic Gardening Catalogue at http://www.organiccatalogue.com/ is one source in the UK that stocks it.
The comfrey roots from my friend are now overwintering in pots surrounded by leaf mulch. I shall be working on a holed bucket for the fertiliser generation in the spring.
(1) The Little Book of Compost: Recipes for a healthy garden and happy planet Allan Shepherd. Collins (5 Nov 2007). ISBN-13: 978-0-00-726727-9