I finally repaired my N gauge Graham Farish Class 25 Diesel locomotive.  I took it apart when it first arrived and would not run (in this instance I think a wire had come loose).  Apparently I didn’t put it back together quite right.  What I neglected to do was to make sure all of the copper conducting tabs (between the bogies and the chassis) were below the chassis (and therefore able to maintain a conducting contact) and not captured inside.  It turns out that the locomotive will run reasonably well with one of the four tabs out of place but will stall on bends.  If by some chance you have made the same mistake as me you may find you need to bend these tabs slightly with a small screwdriver to restore contact.