(These are exceptionally good potatoes from Jersey. Jersey is a Dependency of HMQ, a lot nearer France than England).
Anyway, the season used to be very short (a few weeks at most) and now I'm seeing them six weeks after they first arrived. What is going on?
Are you talking about new spuds, or a small supply of a type of spud are are usually all sold in a short time?
(spuds can last a year when stored correctly)
Chances are your local grocer has caught on to the fact that people like them and they can make more money by stocking them longer. Often grocers and other store keepers go with the theory that certain items should only be sold for very short periods of time to make them seem more precious, but that theory seems to be getting out dated.
If you want new spuds for a longer time, you plant several times.
Early to late, then plant a main crop after each harvest.
(climate allowing)
"Jersey Royal" is not a variety of potato; it is a trademark. Jersey Royals have EU "Protected Designation of Origin" status (like "Champagne"), so the name can only be used for potatoes grown on Jersey. However, the actual variety is called "International Kidney" and is grown elsewhere, including Scotland. In New Zealand, the variety is called Jersey Bennie.
Jersey growers swear that their unique soil & climate make for a special potato. Some growers use seaweed as a fertilizer, which is supposed to improve flavor flavour even more, but it's also more expensive.
According to the Daily Mail, there is a serious shortage this year.
Of course, no grocer would buy International Kidney potatoes from elsewhere and label them as Jersey Royals.
I looked up how the potatoes are grown. The harvest season is traditionally February-June. Potatoes harvested in Feb & Mar have been grown under glass & are in limited supply. But April-June is still more or less a 12-week season. Maybe they just haven't been shipping as much to the wilds of Scotland--probably because they can command better prices in London, and because of the added shipping costs.