Thanks to everyone for their help and comments. In the end the solution was close to home as my son (an N-guager) realised he had one in his collection that may do and it turned out to be a near perfect fit. Some interesting comments about the Airfix auto-tank here; my own experience is that in its early days (when they were new) I replaced the grooved traction tyre wheels for regular smooth ones and had no trouble at all running 2-coach auto-trains up my gradients, which it is doing once again after all these years. It still has its original 6 plunger contacts (but I did get hold of half a dozen spares sometime so must have had concerns about them) and now newly cleaned after a 10-year gap runs very smoothly at a snails pace. Of my other Airfix locos, the tender drive Scot, Castle and Fowler all run much better with the central traction tyres removed (less wobbling and less derailing but with no apparent loss of pulling power), and it was the Prairie that I always had trouble with and still do - very erratic runner!