Us poor Canadians with our obscure early tandem braking systems...
I'd say you're looking for SP2482 (page 3) or SP2485 (page 6) - both are rare rebuild kits. SP 2381, SP1967, etc. kits are more common but googling pictures of their contents, they don't appear to have enough bits in them so they're probably for single circuit brake systems. You'll need to measure the bore of your master cylinder to figure out which one you need. The Girling book link (thanks for sharing it!) says...
SP2482 is for front discs, 8" rear drums, servo, tandem M/C with bore of .812"
SP2485 is for front discs, 9" rear drums, servo, tandem M/C with bore of .875"
My Austin 1800 has a very similar master cylinder but mounts directly onto the servo so that end is different. (same innards) I bought kit SP2482 for it from northwestclassic.co.uk for £17.00 plus £11.40 postage in 2016.
My Epic has Girling drums both front and back, no servo, and a tandem master which looks the same style as yours but .7" bore... not listed in that Girling book. It's rusty inside and out from sitting empty for decades and the only NOS one I found for sale was $370+shipping from Australia.
So I'm going to try a $40 Spitfire tandem MC with the same .7" bore, same brake line threads, same mounting holes. All I have to do is bend some brake lines as the ports are on the opposite side. (facing the clutch cable instead of facing the engine)