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Actually, Bill Scott is probably the greatest living authority on NCC locos. He's always delighted to assist.

I'd splice the wiring back even temporarily in case you've cut a common return to the decode. I can't remember the 071 wiring diagram and although I have both these locos I' haven't looked inside in years so take this advice with a pinch of salt for now. 8113 has 4 newer LED markers. Iirc, the four prisms enable the light to be refracted to the outlet but I can remember where the LED diode is. They're usually covered in black plastic to limit light leak. The older 3-marker arrangement would refract from either a white or red diode but since they have separate outlet , it does seem like there should be a third prism for the red marker....  Looking in the DCC section forums may be of help  

No I hadn't - I'm clearly not trying hard enough! Many thanks for this.

That's very useful. Many thanks. And a good reason to get out and about rather than just staring at photos!

I'm sure you'll have seen the basic description and specs on Wikipedia https://en.wikipedia.org/wiki/BNCR_Class_N BNCR Class N Type and origin Power type Steam Builder Sharp, Stewart & Co., Manchester (No. 42)
NCC, York Road Works, Belfast (No. 16) Build date 1874 (No. 42)
1914 (No. 16) Total produced 2 Specifications Configuration:    • Whyte 0-4-0ST  • UIC B n2t Gauge 5 ft 3 in (1,600 mm) Driver dia. 4 ft 0 ⁄8 in (1.222 m) Wheelbase 7 ft 9 in (2.36 m) Length 25 ft 3 ⁄8 in (7.70 m) Width 8 ft 4 in (2.54 m) Height 11 ft 11 in (3.63 m) Axle load 13.80 long tons (14.02 t)+
17.25 long tons (17.53 t) Loco weight 31.05 long tons (31.55 t) Fuel type Coal Fuel capacity 15 long cwt (760 kg) Water cap 600 imp gal (2,700 l; 720 US gal) Boiler pressure 130 psi (896 kPa) Heating surface 661.12 sq ft (61.420 m)  • Tubes 590.88 sq ft (54.895 m)  • Firebox 70.24 sq ft (6.526 m) Cylinders Two Cylinder size 16 in × 22 in (406 mm × 559 mm) Valve gear Stephenson Loco brake No.42: Hand brake
No.16: Steam brake Train brakes None Performance figures Tractive effort 12,707 lbf (56.5 kN) Factor of adh. 5.47 Career Operators BNCR NCC UTA Numbers 16, 42 Nicknames No.16: Donkey Scrapped No.16: 1951
No.42: 1925 The Five Foot Three reference might yield some assistance. I don't have a copy but someone else might ... Scott, W.T. (January 1968). "The Shunting Tanks of York Road". Five Foot Three. 4: 2–10