I think it's Doves foot Cranesbill Geranium Molle. It ought to be easy to get rid of with a usual lawn weed killer

I think it might be Parsley piert [Aphanes arvensis] .inconspicuous green flowers

Thank you for your replies. We'll go with Parsley piert

I'm sure you are right, Sbg. I can't see any little pink flowers & the leaves are smaller than the Cranesbill. I sometimes leave the latter if it's in an appropriate place, as it is quite pretty.