Thanks for the replies.... I will have to do more reading, obviously
It's quite a common plant near my home, I have 4 plants in my garden as well and most allotments have a patch of it...
We seem to be coming to the end of our summer here, the night time temps are getting dangerously low (4*C last night) I've been using comfrey tea all year, with no other fertiliser - I have nothing to compare it to but my plants seemed to have done ok. Next year I will try a comparison and grow 3 plants, unfertilised, comfrey and a normal chemical fertiliser (if I can find the space in my greenhouse)
I would say that I'm not dismissing the advice but there's a gardening programme on TV here called Beechgrove Garden and they did exactly that comparison last year and the comfrey plant was the most productive. This also might explain why it's all over UK gardening forums