Most experts on diabetes advise against buying foodstuffs supposedly made specifically for diabetics. They're invariably expensive and contain sweetening agents that might result in your spending vast chunks of your life in the toilet! That is probably why Boots have stopped stocking them.
Use ordinary marmalade...but sparingly.
(I'm a diabetic.)
Waitrose do own brand sugarfree fruit spreads, including Seville orange, and also stock that fancy French brand whose name escapes me for the moment - St something-or-other, I think. Much more expensive than the own brand, though, and no better.
My grandma has products incuding diabetic marmalade from Holland & Barratt's health food shop. Supposed to be really good :-) They are nationwide so you should be able to locate one close by. All the best.
Please click here, Sydmane, for a joint website from the Foods Standards Agency and Diabetes UK. They make it perfectly clear that - no matter how many shops may sell you special 'diabetic' food - there is no need for you to buy it whatsoever. If these two organisations don't know what's best for you, who does? Of course, if you're intent on buying the stuff, good luck to you.
Just for the record - I am not diabetic, and neither is my wife, but we buy the Waitrose sugar-free spreads simply because they taste better. They're not sickly sweet and have a much more pleasant consistency than most jams & maramalades. And while it may not be necessary to eat sugarfree products, it can't do any harm, can it? And I've just remembered - the French stuff is called St Dalfour; too sweet for my palate, though.