Before planting comfrey in your polytunnel it is important to remember that once planted, it will be difficult to eradicate it from that spot. Comfrey has extremely deep tap roots and will regrow from even the smallest section of root. That said, there are some reasons why you may wish to devote a corner of your polytunnel (or somewhere nearby) to growing this useful plant. Comfrey is extremely good at collecting nutrients from deep in the soil. Chopping a dropping comfrey leaves, using them as mulch or turning it into a liquid feed can return those nutrients to the surface soil, where they can be taken up by other plants. Comfrey is excellent as a feed or mulch for tomatoes and many other fruiting plants and crops. It also produces pretty flowers and is a good bee plant.
The variety of comfrey most commonly grown in UK gardens in Bocking 14, which is a sterile variety which will not produce seeds. It is most commonly grown either from crowns or from root segments from existing plants. These can be purchased online from a number of different sources and will grow readily and quickly in a variety of different soil types and conditions.
Take the root sections and place them horizontally into the soil at a depth of around 5-10cm in the soil. Remember when spacing that the plants can grow to a fairly large size when mature. However, if you harvest your comfrey then it is not going to get out of bounds and you can keep it contained as long as you are sure to choose a sterile variety so it does not self-seed prolifically. Ideally you would plant out your offsets in March, April or May, though it is possible to begin to grow comfrey later in the season too.
Comfrey can be chopped back almost to the ground, usually twice a year. It is better to wait a year or two, however, to allow the plants to become established. There are many things to do with the plants that you have chopped down. You can, as mentioned above, use them as mulch for your edible crops, especially those which need potash, which comfrey is good at accumulating. You can add them to your compost heap as a compost activator. You can make 'comfrey tea' by rotting down the leaves in water for a stinky yet nutritious brew for your plants. Comfrey can also be used as feed for hens and other livestock and is said to have some medicinal properties for humans.