Stewart-stick with the 60's era theme like myself you are trying to represent the final few years of the ex GNSR lines which makes it really interesting!
Take it you'll be adding a 20 before long then.....
Why not try an EM project anyway? The next project (a joint effort with Neil of this parish)will have the same criteria as you've mentioned above with recycled or unused materials. Will the excellent results you've achieved from this layout, the world is your oyster!
There's a picture of D8033 ,GFYE, ex-works at Crewe on 26th January 1969 p23 Class 20's in Colour by Andrew Fell, Midland Publishing 1994, so there's a good chance that Ian's hunch is correct
Thanks again for your persistance-any clues as to the above?? And don't forget the pic of D5072 at Steele Road in Spetember 1968 in Diesel Days Scotland