Bu##er that's my fault for being too free and easy with the copy and paste, I was trying to link to this http://www.flickr.com/photos/ingythewingy/6169977699/ as it shows the stairs arrangement and the standard window pattern which I can see you've found with the Wellingborough picture, the extension on the Down end of Camden No1 appears to be a relay/battery room for the associated colour lights. I've a soft spot for the Type 13 as my first job on the railway was at Watery Lane Shunt Frame (Tipton)http://www.flickr.com/photos/50619197@N07/4863688894/ , a 50 lever example much reduced from it's heyday but still with a hint of it's former glory.
It's a pleasure to view your progress Iain
Simon