Creeping buttercup is awful stuff. I'm told it is much more common with horses on the land as well. Liming does work, but it is slow. Rosemary had some figures on how much lime to apply per acre, and how much it will change your ph by, but I can't remember those figures off the top of my head. We try and lime every autumn, and sometimes in spring as well. Fertilising is good too- we just spread some rotted down manure. You can also harrow to try and get rid of some of the buttercups as well. One thing I noticed, is when we had sheep on our field , and they keep the grass trimmed a lot further down is that the buttercups didn't get a chance to flower, so at least they weren't producing more seeds.
Beth