I thought that Lincon and Winchester were quite difficult to find in general ... I considered Durham and Bury St. Edmund more common .... Depends on which issue you are talking about. Not all mints struck all of the time. Durham is quite common in later years where the local bishop was granted powers to strike coins, but quite scarce in the Norman period. Bury St. Edmunds comes and goes throughout a 300 year period. Winchester was the old capital of Wessex and a major centre. A few kings were crowned there, and unsurprisingly a good number of coins struck too. Lincoln is for the most part common, but there are still periods when the output reduced in size, leading to scarce issues and corresponding moneyers. Even London has its rarities This is the big drawback with the standard general references. There are many instances where the general description doesn't reflect the populations of the various issues concerned. Take Cissbury for example. In a collecting sense it is a single issue mint. 39(?) coins known, all bar one of which is a Last Small Cross with a dozen or so in private hands. The price given in Spink for a VF example from the mint is £3K, which reasonably accurately reflects the prices of recent transactions.There is a single Cnut Quatrefoil known which is in the BM, so what price a Quatrefoil should another one appear? Guarantee it will be more than £3K.