Mmmm, I've set out a few times to make a JSAA style, as it's one of my favorites. I've ended up with several really nice beers, but none have actually been the same as the James Squire.
Recently I went down the path of upping the crystal malt and willamette hops, as those were two ingredients I knew they used and assumed contributed the most. I ended up using 500g of crystal and 60g of willamette (20g @10, 20g @ flameout, 20g dry to primary).
What I found was that 500g crystal is too much for a 22l batch, and starts to taste a bit astringent and coarse.
Willamette was good but not overwhelming. However, this batch was bittered with 40g Halltertau (8%) so ended up being quite bitter. A really nice English Bitter style beer (wife said it was best ever) but actually nothing like JS... totally lacked the nutty, malty sweetness.
SO... what grain would you use to get that nutty, sweet flavor the JSAA has? "Carapils" has been mentioned here, but may be just a name for something made specially for Malt Shovel brewery...
I usually buy malt from Country Brewer. I've used their "Caramalt" in the past, and also recently got some "Munich" which has a really nice sweet biscuit like taste.
Finally, "Northdown" has been mentioned as a second late addition hop to consider. Can anyone confirm this?