There are selective weedkillers which kill broad leaved weeds like this while leaving grasses alone. You could use one of these.
The only problem with this is that you are treating the symptom and not the cause, and lawns invaded by broad leaved weeds or clovers usually have the wrong conditions, such as poor drainage or lack of nitrogen.
The way to improve any patch of grass is by regular feeding, mowing regularly several times a week, and making sure that you have good drainage by spiking and brushing sand into the holes.
Personally I think life is too short for all that, which is why we have grass and wild flowers and not lawns!

Does it have yellow flowers? Probably oxalis - this grows in the cracks in my path but hasn't yet managed to creep into the lawn. Don't know how to get rid of it but at least you might know what its called!!

i agree with Cammomile, it probably is Oxalis, definately a real blighter once it gets going. I tried Verdone lawn weedkiller. managed to get rid of a small clump, but I used it on the clump regularly until it got the message.