The link to Heywood wood got me digging out my copy of Marl Smithers's book. .
The loco drawings are convenientluy in 1/24 for Gn15, so I can reduce them but the coaches are already in 1/32 scale. This gave me an idea, as I have the Academy plastic kit for Jananese Loco and 3 coaches in 1/45 scale. The coach sides looked about the same height as the Heywood coaches and I was right. It should therefore be possible to build minimum gauge coaches using these O scale coach kit parts. Makes me think the original Heywood design was a 'reduced' full size. Makes sence given the fuzzy grey line dividing minimum and miniature.
The downside to this scale is lack of easy to find cheap motor chassis. I already have some old German models, one 4 wheel diesel, and 2 small 0-4-0s . I think the steam loco chassis might fit one of the smaller estate locos.
Wonder if Tom might consider reducing his Gn15 loco bodies. I know this can be tricky. I can knock up something easily myself otherwise.