Liquid Fertiliser

I understand it's fine for everything. You dilute it to the colour of weak tea. It will stink to high heaven so you could put a lid on it 

Comfrey leaves are excellent though and it's a shame you've not got room for one. The Bocking 14's are well behaved plants

Not really the same thing, sheep manure is high in N but average in P & K.
Comfrey is very high in K and low in N & P.

I think so.

I have only got a half plot - all they could offer at the time, but I have a square 4 feet given over to Comfrey and that is enough for a plot which is 10 x 20 square growing feet.

You don't need much room for Comfrey and the returns are amazing. Try burying a few Comfrey roots around the sides of your compost bin; even almost under the sides and they will grow out from the bin's bottom edge, using no room at all.

I say give it a go - you can always find room for Comfrey!

put your nettles in an onion bag and leave in a water butt , 3-4 weeks later youll have some great fertilizer , as long as it looks like weak tea you should be fine