Hi there.
I have looked through our extensive collection of books on pottery including the Geoffrey Godden one mentioned above, and although there are several interesting references to John Mitchell there are no marks recorded for either Mitchell. So it seems highly likely that they didn't mark the pots. John was taken to court in 1736 by another potter, Ralph Shawe, for infringing his patent, which was on the type of body used for his pots. The judge found in John's favour saying "Go home, potters, and make whatever kind of pots you like". One of the books refers to John dying in "reduced circumstances".