For anyone plagued by mare's tail in their garden, I have learned from a new friend in a local gardening club that it can be put to good use if you pull it up and dry it out then make a concoction.
Place 250g of the dried weed in 10 litres of water and bring to the boil. Simmer for 30 minutes and then leave to infuse for one night. Strain, then dilute 1:5 in water and spray on plants susceptible to rust, scab, mildew and some viruses. It's also supposed to be full of minerals so a good fertiliser and growth booster for young plants.
I haven't tried it myself but all this rain means I do have a whole new infestation in one of my beds and, since it can't go on the compost heap for fear of it spreading to other beds, I will be putting it to the test instead of putting it in the bin.