Found a few of these clumps of buttercups growing in a roadside ditch near the entrance to Roxbury Park.
I have tried to match this with the species known from the area, and it comes closest, in my opinion, to Ranunculus muricatus, which is said to be uncommon in your area. The best way to be sure of this identification is by the fruits, which are spiny on the face. There are no fruits in the image, but the size of the petals and shape of the leaves look really good for this species. It is usually called the Spinyfruit buttercup. If you happen to get back to it, see if the fruits are somewhat spiny.
Steve Hill, Botanist, SCNPS