Thank you Martin.
I am content to close this issue now, as I think you have taken it as far as is reasonable, for which I am grateful.
The micro-switches have only two contacts (a third slot aperture on the body of the m/s has no contact emerging), so there isn't any way of changing them to normally open, and the decoder board has no options. But not to worry, as I can live with having to make a manual adjustment after Auto-Detecting the switch operation.
Thanks for your help with it.
Best wishes,
Ken