He observed my handling of the 'box for most of the day, and examined me on points of signalling rules and 'what if' situations. He must have been happy as he gave me this!
So now I'm a qualified signaller on the Churnet Valley Railway. An ambition achieved! What I need to do now is get a few 'turns' in charge of the 'box under my belt as rostered signalman. I suspect this is a job which you really start to learn as you do it for real... on your own!