40120 can be seen at Newport at the head of the F&W railtours 'Gwent Valley Invader', awaiting class 37's 269 and 233 to take the train up the valleys.
This is another image that sixbellsjunction assisted with.
More details here:-
http://www.sixbellsjunction.co.uk/70s/780311fw.htm
Another photo of this railtour can be found here:-
http://andygibbs.zenfolio.com/p483082284/e3ad401d6
Slide taken 11/3/78.
Photographer unknown.