I heard Bob Flowerdew too. If you can find it on IPlayer then it will be worth a listen.
He said Comfrey tea is very good for stuff where you need a high potash feed - tomatoes, potatoes etc. But if used on its own as a general purpose feed for other things can lead to a bit of nutrient imbalance (things start to get yellow leaves). But if you stew together Borage and Comfrey (and Nettle as well if you have some) then that makes a better balanced feed. Or mix together separate brews of the above. He was quite enthusiastic about borage tea as a general all rounder - and said it was very easy to make - just stick a load of borage in a bucket with some water, put a lid on it and it could be ready in a week.
I made nettle and Borage tea and it doesn't have the awful stink of comfrey. Been using it round the plot and nothing has keeled over yet.