Comfrey is just about as easy as it comes , to grow. The best type to get is bocking 14. It is a sterile variety and therefore won't seed all over the place. You start off with a root cutting , I got mine from chase organics, and whenever you want more just dig up a plant , and take some of the thick root chop it into 1" sections, plant, and away it goes . You can use it as a liquid feed , just cut some comfrey stick it in a barrel fill with water and leave for a few weeks . The gunk that comes out stinks to high heaven , but when watered down is a very good feed. You can also just cut it for the compost heap . You can cut the plants every 6 weeks or so and if you put plenty of manure on it , it will provide huge amounts for composting.
Well done with the purple sprouting....that is sooo lovely fresh , and from what I have read , is very very good for you too. If heating oil is still cheaper than parraffin you can use that instead. I tried it in parraffin lamps and it worked fine.