Thanks for the response fellas, I do appreciate it.
I now have a much better understanding regarding the different chamber and cartridge specs that the die makers have to contend with.
Yes, I have seen the sometimes big difference between original Kynoch ammo and fired cases.

I have decided to stay with the set up that I've been using for the last couple of years, rather than push for a replacement die set.

I use a die of my own manufacture to set the shoulder position and diameter so that the brass drops into either chamber with no binding but also with minimum clearance for optimum case life.

For me, little problems like this one involving sizing the cases, are part of the fun of owning and shooting double rifles.
It gives me a lot of pleasure machining double bits and pieces on the lathe.

Then of course the ultimate pleasure comes when you hunt with a rifle that works well, partly as a result of the personal time and effort put in by its owner.