I have reluctantly accepted defeat.
Despite my best attempts to resolve the running issues encountered with the steel rail tarnishing and rusting within hours of running, I finally concluded the project would have to be abandoned. I debated replacing all the steel rail with hi-nickel as this can't rust but removing the steel rail without permanent damage to the track work simply wasn't possible and I'd have to make replacement turnouts as well as plain track work. I'd essentially be starting from scratch once more with blank boards.
Although my workshop (converted garage) is heated, double glazed and insulated, there has always been a slight dampness of atmosphere issue in the winter months and any steel tools stored therein can develop a faint rust haze if not used for a while. I don't like to admit defeat but after many months of random attempts at resolving things, I decided to abandon the model and it is no more.
Ironically , Julian originally intended breaking up the layout to make way for a new project and I've ended up doing the same. I've rescued all buildings, signals, point motors - anything I felt could be used again but the track work was removed, boards were dismantled and taken to the local recycling centre. I still have the support legs as they can be used for the next project, whatever it may be, and the seventeen foot long printed banner back scene has been rolled up and stored for future use. LED lighting and tubular steel supports are also reusable of course.
So that's the sorry end of a rather long and protracted tale, but hopefully I have learned much in the process and my next project will bring a more positive outcome.