Kurtail Gold if you have lots to get rid of but treatment is a bit on the expensive side. Before you invest, buy a weedkiller with glyphosate, mix with a wallpaper paste and brush onto the stems after crushing the stems as they have a sillica coating. This will in effect allow the weedkiller to penetrate the weed. You might have to do this several times. The larger the weed the easier it will be to absorb the weedkiller and send it down to the roots.

You should really call it "Horsetail". Mare's tail is a different plant.

As said above, Kurtail is the stuff to use - although the roots have a nasty habit of surviving even repeated treatments. One way to discourage it is vigilance - every time a frond appears pull it up and bag it (DO NOT ever compost the stuff). After a while the denial of opportunity will stifle it. Although this only works in small garden borders…

I've been pulling it up for a couple of years now - every day, morning and late evening. I won't let it win but I'm not going to win either. It's now coming up between the stone flags in my greenhouse, despite a thick layer of hardcore beneath these !
Am now looking into Kurtail.

Last year I started my fight with Horsetail and I have it in between planted up plants, I now have bought some tubing to place over the offending plants and then spray into the tube so then not getting any weedkiller onto the other plants. This year I have had less come up then last. Last year I on hands and knees very very carefully dug up as much as possible as I found that if I just pulled one head then 2 or more heads grow from that site. In areas where I have no plants then I cut the bottom off milk bottles and pop bottles and put them over the Horsetail, spray then put the lid back on. After about 3 weeks with spraying a couple of times into tubing and bottles the Horsetail spike has died back. I know it will take a few years but am hopeful. I used a weedkiller with glyphosate in it Jen

Although this my second comment I have been looking to see what other gardeners have been using for this problem. There are many reports of gardeners using the following chemical and I wondered if anyone has used it with some success.

Ammonium sulfamate aka Ammonium sulphamate ( not to be comfused with ammonium sulphate ) is apparently one of the very few effective chemicals that kills it ( horsetail ) off but at the cost of having to leave the ground uncultivated for a couple of months.
It is easily available and reasonably cheap in the UK and as it decays it leaves nitrogen as a fertilizer in the ground.

It almost suggests that you would be giving a boost of nitrogen which the plant cannot cope with. I believe someone had flagged this some time ago.Any ideas?

Thants interesting Jimmytheone, as in my LOTS of looking up and I had a local gardener around to quote on a garden fence, I saw some articles and he said to enrich the soil as Horsetail like poor soil but didn't know the nitrogen bit. Don't know anything about this being flagged. Jen