Thanks so much Dungrange for your reply. It was just what I needed.
Yes I know about the resistor - I have used this for my mimic board, so this should be no different. I don't know, but I assume if I buy a proper signal, it will have all this already.
From what you write, I think I need a current sensing device. I assume you think that such a device might well have some sort of relay built in. I will phone CostalDCC on Monday for advice. The NCE makes a little sense as I use a NCE Powercab.
Again thanks.