Kurtail is a total weedkiller and will kill horsetail, weeds and grass in about 7-10 days - this is the only thing that will kill it as far as I know -- Check (progreen.co.uk)
for more information. They sell it £25 a bottle -Ken

I had the same problem many years ago, the only way I could get rid if it was by pulling it up every time I saw it and getting as much of the root out as possible ( but it took a few years to clear it.