Yes, it's a nightmare... (sorry!) In a lawn, regular mowing should keep it under control. If it's growing through paving or in veg beds you can spray with strong glyphosate in late summer, after bruising the foliage first to enable it to absorb more of the weedkiller. Doesn't work well at other times of year - and may take several years to kill it off. Not practical if growing among shrubs etc. Removing every little bit you see, as soon as you see it, does weaken it eventually if you can't get at it with the weedkiller. But it's a real dinosaur of a plant...
Since 2019 I've lived in east Clare, in the west of Ireland.
It seems to be a 'good' year for mare's tail- mine is creeping nearer and nearer the house! I believe that in Germany? it is gathered into bunches and sold on market stalls as a pan scourer.
This vigorous crack-dweller has a dozen names, a sure sign that most people consider it unsightly and invasive. It is one of those "oh, so that's what it is" kind of weeds. Also known as horseweed and, more properly, Conyza canadensis, the mare's tail is prolific in both rural and urban settings and will grow with hardly any water or soil straight and tall, up to 4 feet high. Again, that's a lot of food.
The leaves are most palatable when young. By midsummer, only the top foot or so of a 3-foot plant is tender enough to eat after a quick boil. They are peppery and, in fact, you can dry them as a spice. As with many dark, leafy greens, the plant is a decent source of calcium, potassium and other minerals.
And now for some Boy Scout trivia: Mare's tail is the weed of choice for making a fire via the drill-friction method. The very straight, hard stem rotates perfectly between the hands to make heat. What other plant can make the fire needed to cook it?"
Ah. In that case, although edible, they're not as versatile as the first 'variety'. Apparently in Alaska they dry them and use them for soups, as they have vitamins, but they need slow cooking to bring them out. And yes, actually, they're good as pan scourers, but you have to mind you don't lose too many bits down the sink!
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Yes, it's a nightmare...
(sorry!) In a lawn, regular mowing should keep it under control. If it's growing through paving or in veg beds you can spray with strong glyphosate in late summer, after bruising the foliage first to enable it to absorb more of the weedkiller. Doesn't work well at other times of year - and may take several years to kill it off. Not practical if growing among shrubs etc. Removing every little bit you see, as soon as you see it, does weaken it eventually if you can't get at it with the weedkiller. But it's a real dinosaur of a plant... 
When you find out the answer, patent it and you'll be VERY wealth
It seems to be a 'good' year for mare's tail- mine is creeping nearer and nearer the house! I believe that in Germany? it is gathered into bunches and sold on market stalls as a pan scourer.
It's also edible....
"Mare's Tail:
This vigorous crack-dweller has a dozen names, a sure sign that most people consider it unsightly and invasive. It is one of those "oh, so that's what it is" kind of weeds. Also known as horseweed and, more properly, Conyza canadensis, the mare's tail is prolific in both rural and urban settings and will grow with hardly any water or soil straight and tall, up to 4 feet high. Again, that's a lot of food.
The leaves are most palatable when young. By midsummer, only the top foot or so of a 3-foot plant is tender enough to eat after a quick boil. They are peppery and, in fact, you can dry them as a spice. As with many dark, leafy greens, the plant is a decent source of calcium, potassium and other minerals.
And now for some Boy Scout trivia: Mare's tail is the weed of choice for making a fire via the drill-friction method. The very straight, hard stem rotates perfectly between the hands to make heat. What other plant can make the fire needed to cook it?"
From here.
Other sites available, if you google 'Mare's tail edible?'
Hang on - do you mean this one:
http://eattheweeds.com/wp-content/uploads/2011/08/conyza2.jpg
Or this one?
http://www.greatnorthernprepper.com/wp-content/uploads/2012/05/horsetail.jpg
Ah. In that case, although edible, they're not as versatile as the first 'variety'. Apparently in Alaska they dry them and use them for soups, as they have vitamins, but they need slow cooking to bring them out. And yes, actually, they're good as pan scourers, but you have to mind you don't lose too many bits down the sink!
thanks for the suggestions Kurtail sounds like the one for me