I don't suppose anyone here has a copy of diagram C or something else in the way of works drawings for the 1813s as saddle tanks? I drew pannier tank variants for the book, and the side tanks above, but I don't have anything for the stage inbetween, which seems something of an oversight. NRM has a copy, but while I don't mind paying their reproduction price for a full GA, it seems an awful lot for the minimal info on a weight diagram.

No drawing, but here's 1815 posing c 1905 according to RCTS. It has an original length, 3-segment, tank. 'Admiralty' rods.

RCTS reckons that saddle tanks were common with the 1854s, so that provides a starting point. 

I think the boiler pitch on the saddle tanks, even the earliest ones, was 6'11 3/4".

I've been sent a copy of Diag C, so I'm covered:-)

Yes, as far as I can see once the original boilers were gone the pitch was 6'11.75. RCTS doesn't document the handful with domed boilers and side tanks, but they look as if they are higher pitched than the originals to me, and so I drew it at the higher figure. 

I think the 1813s would be a R2R manufacturers nightmare. I suppose the fact that none lasted long enough to receive BR livery would be a major killer anyway, but you can permutate forward dome, back dome and domeless boilers, round top and belpaire fireboxes, side tanks, five panel and three panel saddle tanks, short pannier tanks, full length pannier tanks, various bunkers, narrow, standard width and enclosed cabs, top feed or not, superheating or not, but of course all GWR locomotives look the same...