Trev,
It just shows how out of touch I am with British N-scale. Until I read your post I had never heard of Union Mills so I decided to do a quick search. What little I could find was good and I was surprised to see that they (or should that be he?) build complete locos and don't just stick a new body on a Grafar/Bachmann chassis.
Don't forget, if you get tired of waiting for a small British loco to come along there are any number of good-running Continental locos out there. I know you hate the idea of running a Swindon-built loco on your layout but surely something from Munich would be acceptable.