Hi Philstutt
I am VERY fortunate never to have met horsetail/marestail on my plot - for all that it may be fascinating to have a pet plant dinosaur.
As far as I am aware, there is no organic way of dealing with it, apart from eventualy exhausting the root system by systematically removing all signs of it - not just on your plot, but on the whole area in which the plant appears.
I hope I'm wrong - so do keep looking and asking. If it's any consolation, as it has a thick shiny skin, precious few chemicals deal with it either!
Sorry - Gavin :-[