We have quite a lot of horsetail on our allotments,and we give this
advice to plotholders.
1)In May/June whenever you arrive at your plot,spend 10 minutes
(obviously more at first)seeking out and digging up the horsetail
growth with small handfork.Horsetail needs light to grow and
continually removing it significantly weakens it.
2)In July/August start applying glysophate(strong mixture mixed up
with wallpaper paste so that it sticks to it).Applying it at the end
of the season is far far more effective than earlier,since the plant
is naturally sending back down into the roots-rather than upwards as
in Spring growth.

Several plotholders have removed horse tail in a couple of years
almost completely.This works.
Michael