Many thanks to those who replied onlist and offlist.
The rim sherd I described has now been identified by John Allan of RAM Museum, Exeter. I should have stated that the excavation is in North Devon.
John writes, "The other interesting piece is the green-glazed white ware sherd with the white muscovite inclusions. This is a Saintonge (SW French) product of the 16th or 17th century. There are similar finds at Plymouth and Southampton but they are generally scarce in Britain."
Have to think again about my context datelines and wonder what on earth such a sherd is doing in a remote part of North Devon at a depth of 2m+?