Thank you gentlemen.
Norwich is certainly one of the less-common mints of the period. I've got a complete mint run for this period, I've just got to upgrade it to an all-VF set.
The really rare mints are Shrewsbury, Ilchester, Wallingford, Wilton, Carlisle, Hereford and Durham. Then there are the scarce ones such as Lincoln, Exeter, Oxford, Norwich and Northampton. Then we move on to the slightly more common ones such as Bristol and Newcastle until we get to the common ones of London, Bury Saint Edmunds, Canterbury, Winchester and York.
Here is one of my Durham pennies:
Whereas Shrewsbury is also very hard to come by: