My cup runneth over. Many thanks for the input folks.
Okay, here's what I have. I've done some better detailed shots of the sprues that are not entirely identified, or mostly identified at this stage. I've left out the regulator, backhead and the firehole bits.
Sprue 1: Oil pot lubricator/hydrostatic lubricator. Choose one or the other, apparently.
Sprue 2: Steam fountain. Superfluous for this build?
Sprue 3: From the top clockwise: vac brake handle / steam+vac brake ejector / clacks (x 2) / unidentified (x 2)
Sprue 4: On the right, [alternative?] regulator stuffing box and whistle valves, with water gauge below. The top left seems to be a handle of some kind, and I wonder if it's meant to be the top of the sanding lever that lives on the front of the bunker. I wonder this, because there's nothing else remaining in the various bags and the box that fits the bill for this fitting. It's unclear what the other two items are.
Sprue 8: Possibly a 3-way W valve?
Sprue 10: Whistles, snifting valve and cylinder draincocks (not required for this build), steam lance cock.
Sprue 11: Safety valve springs and tank filler bump stops.
Sprue 12: Sanding gear lever, cylinder drain valve lever, backhead clack blanking plates.
Sprue 13: Rear sandbox lids.
Unidentified, and new for this round!
As Graham and Simon noted:
I agree re Mason's valves and 5700 engines were fitted generally for steam heating of carriages. Heather, you have some photos of the trade-built engines and you ought to be able to see if there is a steam heat bag hanging under the buffer plank.
I can confirm the relevant lots were steam heat fitted from new.
I'm kind of hoping another contact has a good set of photos of a 57xx cab layout, but I'm waiting to see when he can sort them out for me. Sooner, rather than later, would be nice, though.