One of the best known uses for comfrey is comfrey tea.
If nothing else, it's worth growing it solely for making into this garden elixir. Comfrey is a deep rooting plant and so draws up loads of trace minerals from deep in the soil where other plants can't reach them. Because of this, it can be made into the mineral rich soil improver that is comfrey tea.
Actually, as a little aside, this is one of the reasons that organic food is so much better than chemically farmed food. All these trace minerals are vital to good health in both plants and people. Conventional fertiliser only contains the bare essentials - nitrogen, phosphorous and potassium (NPK) and so this is pretty much all that gets accumulated in the tissues of most of the food sold in supermarkets. So using mineral supplements on your soil, such as comfrey tea, makes sure you're getting more of those important trace minerals from the food you eat.
Apart from all of these trace minerals, comfrey is especially good at accumulating potassium. This is a nutrient needed in particular by fruiting crops, so comfrey tea is often used on tomatoes, strawberries and beans. It also accumulates nitrogen and phosphorous and weight for weight contains more of these nutrients than farmyard manure.
To make the tea, simply harvest and chop down some comfrey leaves and almost fill a bucket with them.
Cover with water and add a well fitting lid. This last step is very important as it will smell pretty bad by the time it's done!
The best way to do this to avoid contact with the smell is to use a bucket with a tap and then put the comfrey in an old sack to soak it without blocking the tap up. I sadly don't have that kind of equipment here, so have to endure the smell!
You leave it for around 3 weeks and then draw it off for use. Make sure to dilute it - at least 50:50 with water or up to around 1 part comfrey tea to 15 parts water. This can be used either directly to the soil around your plants or as a foliar spray. But stop spraying at least a month before harvest.
Interestingly for me and my mildew problems, the spray is also said to be good for powdery mildew. Might have to give that a go on my gooseberries!