I had loads of creeping buttercup in my garden when I moved here about 6 years ago. I just pulled it up, and treated the lawn. The trick to pulling it is to get the roots - they are long and white, so are really easy to see. It's very therapeutic as you can start pulling up one bit, and the roots go on for about a foot, or onto another plant, so it's easy to get up a lot at once. The roots tend to grow quite shallow , so they will come up quite easily. I rarely get it now, but when I do, I just pull it up. It's fortunately one which responds well (or badly, if you're a buttercup!) to feed, weed and moss killer, and as
@Wiebke says, as long as you cut the lawn a few times before you let them back out, all is well. I wouldn't worry too much about the odd bit though, just keep on top of it, and it will soon disappear. I also weed through the fence into my neighbour's to try and get it before it gets to my garden.