Lawn weed killer is what I use. Not fond of chemical warfare, but not easy to eradicate anything from within grass without damaging the grass itself.

Unless the infestation of weeds on your lawn is particularly bad, I would wait another month or so when the weather cools off. It wouldn't hurt to water the lawn regularly if you can so that the grass will strengthen up. A week or so before you intend treating the lawn, don't mow it so that the Yarrow puts on plenty of leaf to take up the selective weed killer lawn treatment. In the meantime, if you can, remove the flowering heads of the Yarrow in between mowings.

Hi Ponty I remember you. As already said if there is a lot chemical warfare is the way forward. letting it grow a bit first as previously advised. if only a few dig them out using an old knife in a circular 'gouging' motion.

Thanks, everyone. This is a new problem for me and has been "driving me up the wall" this year. I think it originated on a nearby green.