Alan, your hunt noted.
Suggestions, but shame Ken Nunn collection did not get into the public domain, this happens time and again.
Try the HMRS
Try County Record Office for the area in question, its surprising what they have.
If you know the makers of the locos, e.g. Sharp Stewart, Beyer Peacock, and see what records are around for those, again the county record office for the area of manufacture might hel
If there is a local newspaper still running in the area of operation, ask them to put an appeal in. This is quite event now, although used to be fairly commonplace, so would likely be done.
I am well involved with the Narrow Gauge Railway Museum at Tywyn, which covers the entire UK & Ireland narrow gauge scene, i.e. from standard gauge to 15″. I can enquire what material we have, know there is some on 3′ gauge Irish.
Regards, Gerald Grudgings