Update... Since your clearances are much wider, I got worried that the frame member might be bent inward. So I took a closer look and all of the frame parts seemed way too close to the rotor. In some places barely a mm of clearance at the very edge of the rotor. So I double checked that there weren't any axle spacers or anything like that getting in the way... nope, there's no way the cassette could go any closer to the right-hand dropout.
Then guess what I found:
YES - A 3mm DISC SPACER!
So my mtb frame must have had the disc mounts slightly wide, and the BD slightly narrow, and trying to use the same wheel clearly doesn't work!
So removing this, there is just enough width to get the older CPS mount in. The newer ones (I have a spare for the front) are definitely a bit narrower. So it looks like all I hve to do is get a minor refacing to the mount holes and a new brake will jusssst fit.
Thanks for all of your help and measurements in this thread guys... disaster averted!