Hi Peter, only just caught this. Try looking at Rocky Mountain Cartridge brass, Okay it has to be imported from the US but a quick e-mail should establish if the 12g brass is equipped for shotgun primers (their .410 brass is), price list here:
https://www.rockymountaincartridge.c...oad%20Kits.pdf
Worth a look if avoiding re machining faff is an issue.