I agree Dave is making a really nice job of this, I am a fan of all those extra bits of plumbing as some up and coming posts of my own will show.
In terms of how much lateral clearance you need over the centre axle Dave, I think I can help a bit. If you read through the first page of my Track Gauge Widening, All You’ll Ever Need to Know thread
, you will find that, as a by-product of the original discussion, I generated a spread sheet that will tell you all you need to know to get a locos with a variety of different wheelbases round any given curve. This separates out the effects of any gauge widening, the extra clearance you get courtesy of the P4 standards (as opposed to the S4 exact scale ones) and the impact of a bit of side play on the middle axle. Near the bottom of the page, the Spread sheet is available to download if you fancy having a play with it.
Dave Holt wrote:....The instructions warn to be careful not to distort local to the cab spectacle windows when folding the sides. This seems virtually impossible due to the lack of material to hold. So, despite using a hold-and-fold tool, the front plate did distort and had to be pressed back to shape afterwards...
Never had a hold-and-fold as I learned how to fold etched brass before they existed, and I remain happier with my own methods which involve flat surfaces a steel ruler and a craft knife. You do have to be careful to do it right to avoid the sort of distortion you describe. This means that the bit with the hole close to the bend is held firmly against the flat surface with the ruler and the bit away from the hole is bent up. I presume something similar is possible with the hold and fold. However, it is made much easier if you deepen the fold line with a triangular needle file, until a witness line shows through. You get a sharper corner as well, but only one go at making the bend as it will snap if you try to bend it again. This tip I learned from Danny Pinnock for doing the long folds on his D&S coach kits. I do wonder what it does to DaveBs bend allowances though.