Ulfhere-

Good.
My first comment would have been something like
'fire form a case from each chamber, then check your H20 volume'
but I figured I'd do the standard double rifle reality check first.

Don't expect the chambers of any two (insert name of whatever brit express cartridge here) chambered express rifles be exactly alike -- anything could have happened or been requested back when the rifle was built - or since then.
The only way to know for sure what's going on in there is to find out yourself.
Pop a couple pieces of brass with your favorite cream-of-wheat recipe for the water volume and slug the bores for bullet size.
That'll clear up a thing or two while your cerrosafe order comes in from Brownells.

You might get lucky and be able to fire-form readily available brass, then neck size from there while you wait for the custom dies to come in from RCBS or CH4D or wherever you choose.

Some guys get lucky and hit a regulation load on the first try.
You might even bag some game with the thing while you're at it.
Stranger things have happened.
I ripped the brain stem out of the first piece of live running fur that I pointed my Tolley at (rifle in my signature line) one day on the ranch while working out a regulation load.
I wish you the same kind of luck, but on something bigger and more appetizing than the skunk I killed that day.

Cheers
Tinker