Thank you Spule 4, and all. You obviously have that rare gift of "key word" divining to have located that little nugget. Would seem to be the best possible cause as the loco, while running under load, was in no way speeding.
So here is the plan of action - cancel trip to Maplins or request for replacement PCB's from Bachmann, learn how to use my multimeter in future (not just track voltage tests), switch transformer to lower voltage range, fit the little salvagable brown bit between the terminals to suppress noise (done), double check no binding in motions or gearbox and pray no permanent damage has been done to motor. Perhaps make a check on my other Bachmann loco's to see if this little device needs removal there too/
All put back together (make sure no trapped wires to eventually short) and run on rolling road. Seems fine, no nasty noises or smells. Time to pop out in the sun and give the 10.00 varnish the highball.
Quite an education, thank you again all. Max.