If you are as stupid as me, you don't always pay attention when dismantling a model locomotive.
If the loco concerned is an Electrotren and you try to pull the baseplate off, you'll stretch the wires that pass up through the cast chassis. This allows the insulation to pull back and the positive and negative electric juice to escape, meeting each other in a sparky way that DCC controllers just don't like.
If you saw the model at Doncaster you'll know it wasn't running. For Warley I decided to fix it, partly prompted by the promise that the manufacturer of the resin body would be visiting.
Anyway, I carefully unscrewed and unclipped everything with the results you see above. Apart from some fiddling popping the waggly bits back into the slide bars, it's not too bad a job. I rewired with the thinnest wire I had to hand and double insulated the exposed wire at the top of the pickups with blobs of superglue - just to be on the safe side.
Once reassembled, the model runs sweeter than such a cheap chassis has any right to, especially one with such a cheap 3-pole motor. I've kit built chassis that don't work that well and they have "proper" motors in them. Grrrr.