Thanks for the comments, much appreciated!
But here's something not so great, so I'll get it out of the way, that I've decided not to use the replacement fuselage.
I was well up for this, after reading all about the error with the Roden Triplane fuselage. The consensus was that 6mm had to be accounted for, and that the error may have been derived from the prototype having been shorter than the production planes.
Brian Fawcett did a brilliant job of providing a replacement fuselage, incorporating several other improvements thanks to his being a nifty sculptor. The fuselage latticework is especially pleasing compared to Roden's vaguer rendering.
IIRC this was all just before WNW's Triplane appeared, and I guess not so many of the Roden version have sold since then. But I had it, along with the resin fuselage, stashed away until now.
When I compared the two fuselages, I was stupefied. If you look at the picture, you can see why. Yes, the Roden fuselage is the grey one.