Possibly a Cerastium (Mouse-eared Chickweed)? Probably need to use a selctive weedkiller on the lawn.

Many thanks

Check out pictures on the internet, Borodin, before you accept my thoughts ;-)))

Yes, Bulba is right, it's Cerastium fontanum.

You are both correct. It is Cerastium fontanum. I am currently treating it with Weedol Lawn Weedkiller which appears to be having an effect. I am reluctant to dig it out because the lawn was only laid just over 18 months ago. I may be forced to reconsider though if the weedkiller doesn't work.