Whilst it is true that Scottish industrial concerns tended to favour local builders there are hundreds of examples of English industrial steam locos, of all sizes, in Scotland, acquired both new and second-hand, and the biggest single supplier of industrial diesel locos was Ruston & Hornsby, of Lincoln. However, this photo was not taken at a Scottish industrial business, it is at a site being used as a military base in the middle of a World War. The War Office requisitioned locos from all over the UK and sent them wherever they were required. Of the four other locos at Invergordon, three were "Terriers" from the LBSCR in the South of England, and the fourth was the Hull & Barnsley loco shown in this photo, so it would not be surprising to see a loco from Co. Durham there as well.
The cab has been extensively modified, although it does incorporate the original weatherboard (you can see the join in the upper cab front, just above where the safety valve lever enters). The lower side sheets may even be replacements fitted by Hawthorn Leslie, as they are of their pattern, and are shorter than the original I'Anson sheets, which stretched to the saddle tank. The cab steps are original, and provide the best indication of the builder. Offset steps were common on 19th century tender locos, but far less so on tank engines. Relatively few builders fitted them, but examples can be found from Beyer Peacock, Black Hawthorn, Robert Stephenson and Sharp Stewart, and also the North Eastern Railway (Gateshead works), and probably others as well. However, the only builder known to have fitted this cast pattern with the triangular cut-out was I'Anson. Taken in conjunction with the wheels and the other design features, I'Anson ticks all the boxes and any differences could be explained by alterations made during the course of what, by the date of this photo, would be a 40 year life span. It is a closer fit than any of the other builders suggested in this thread, although whether it is the South Moor loco, or another, possibly unrecorded, example is, I agree, open to question.