I have not got a Hornby Castle in front of me as Philip has, but point out that a five mm difference is 'only' 2.5 mm each side. I don't suppose you inadvertently transposed 12.7 for 17.2 Philip?
A 12.7 width would make it narrower than the frames (should be 16mm in accurate 4mm scale, appreciated that Hornby are modelling at 3.5 mm scale in that area , so 14mm). What are the frame /footplate dimensions on the Hornby, may I ask?
Given the trapezoidal tapering shape of the Swindon firebox, visually the important factor is the extent to which the cladding representation is seen to widen from the cab forwards to the join with the barrel cladding. I have seen models that did not look sufficiently tapered in this respect. We must remember that the frame inside width is just over 4 ft so the lower firebox beneath the frame has to fit that dimension, if one is going to model any of that. Not even Guy Williams did much between the frames IIRC (think two sets of Walschearts valve gear) and those items are far more visible.
By the way, in 4 mm dead scale that leaves just some two thirds of a mm between frames and wheels!
A correction to my earlier post in that by reviewing actual measurements in ft and ins, given on drawings, it shews that the footplate is 8ft 8ins wide (so indeed Hornby at 35 mm on my Saint are fine). I was falling into the trap of measuring the diagram pictures in books instead of reading the actual words and numbers on the drawing.
If it looks right it is right IMO. It's more important not to have a 1925 era representing model having G badge W on the tender ( I see that type of error often at shows).