A few hours spent browsing my stock of books on the GCR on Sunday afternoon produced a working hypothesis regarding the panelling/ matchboarding problem.
Broadly, I think there are three variants of brakevan that were produced practically at the same time and all to the same basic design.
-1907/08 panelled full brake with corridor connection (as drawn in Volume 3 of 'Great Central')
-1907/08 panelled full brake, no corridor connection (as shown in a photograph in the same volume)
-pre-1910 matchboarded full brake, corridor connection [?] (appears in several photographs, but never in very good focus- there is definitely 'something' vertical going on below the waist however).
So with that provisionally solved I'm staying true to the drawing in the book, and these last few days have been experimenting again with teaking.
This time I used a gloss yellow (Humbrol #7) as a base- two coats. Then I used an old brush and put a very thin, streaky coat of mid-brown (Humbrol #133) over it, and then removed most of this with tissue paper. I then worked back over it again with the brush- this time with the brush practically dry.
I think this has worked quite well....
I'm now giving some serious thought to repainting my existing GCR stock in similar fashion...