This one is creeping buttercup, Ranunculus repens. It is stoloniferous, hence its name. The sepals differ from bulbous buttercup in that they are spreading and not bent back on themselves (reflexed).
It is very common and probably most easily identified from its leaves, especially when differentiating it from the third common buttercup (see tomorrow), meadow buttercup.
I didn't realise that all buttercups are poisonous to animals. This no doubt accounts for their ubiquity in pasture. This toxicity disappears, however, on drying, rendering hay safe.
In the top left picture you can see an achene, the cluster of fertilised carpels, which are characteristic of the buttercup family.