That's great. Not sure how I missed the Barrowmore book. I've also found GM/RC2513 from RSSB which used a Deltic to to set the standard for all future P2 calculations. (D9001? at Didcot in the early 1970's on dipped rail joint tests if I recall). This give an un-sprung mass of 1680 kg/wheel which is only 60kg/axle higher than MT25. This seems to equate to an wheelset weight of 1.3 to 1.4 tons including the spur gear. The US SD45-2 has a measured wheelset weight of 1.5 tons.
Thanks for the help!