It was just a guess, but he works in reduced stoneware so the colour of the clay will be much the same as on yours and the brushwork is similar.
I'm also pretty sure from you photo that a bit of the mark has'nt come out, which is common with impressed seals as they can get clogged with clay from the previous pot. There is definately a different surface to the mark from the top of the 'I' to the middle of the 'P'?
Anyway I can say for certain it's not by Pincombe (although that is'nt a huge help I'm affraid)