Ray mears wild mentions four members of the horsetail family Equisetaceae.
Wood horsetail [E. sylvaticium] it says" the green parts are poisonous: in fact he only part of the plant that is definitely edible is the bud at the end of the rhizomes, which starts to swell in early september [book gives picture of this part of the plant].... Steaming or boiling them ..the buds have a soft floury texture and taste of potato with a hazelnut aftertaste.
Marsh horsetail [e. palustre] No mention of toxicology for this plant, but the buds can be eaten the same way as above.
Great horsetail [E. talmatia] wetland plant. The buds start to surface in january... The buds are not pleasant and are possibly poisonious but the sporagenic vegatative buds taste pleasant. (I don't quite know what he means by differance between the two types of bud) They are small and produce a small yeild, so are really not worth the considerable effort involved in getting them.
The same proberly applies to the sporogenic buds of the field horsetail (E. arvense). However it is worth mentioning that the dena'ina of alaska use what seem to be the sporogenic tubers of this horsetail as a food in their raw state. { long piece about the indigenous consumption} .. Clearly we need to test some populations of british field horsetail to see if any of them are equally edible and have the same properties. In the meantime, we recommed that you don't try to eat the sporogenic buds of british populations.
Horsetail [e.arvense] tea is mentioned in quite a few herbals as a nutriative tonic and as a salve for rheumatism. In alaska it is eaten at the begining of spring as the snow melts, the natural diet during the winter is very high in meat so is high B vitamins. Bare in mind some chinese medicines produce liver changes in indo-europeans, but are fine when used by amer-asiatics.