Thanks Shaun, this is what I intend to try. I've been looking at IRDOT to detect the train, this can then throw the points and the auto-reverser 'should' take care of the polarity change. I'm going to order some bits and have a try in the shed to see what happens. The loop is planned to be close to the house and I've got some watertight plastic cases, that we've used for outside Christmas lights, which are already mounted on the wall close to my intended route to house the electronics.
I've been working in the shed today, creating an area for the fiddle board and clearing vegetation for the likely route of the track. It looks like we've got honey fungus and sadly a lot of the woody plants are dying. Bad for the garden but good for my route clearance as I'd never have got permission to cut them down!