It is one of the chickweed family. Depending on how much there is you could just pull out by hand. Spot spray with weedkiller or a lawn feed and weed type treatment.

Thank you very much. I’ve been
trying to pull out bits of it but the root often doesn’t come out. Do they set seed as well so is it still worth doing?

yes they certainly set seed, so definitely worth doing.

Thank you - I shall persevere !