Just got an update from Rourke Cycles. Turns out my bike IS a Rourke (and not Mercian), confirmed by Brian himself:

Brian has identified the bike, he says the frame was made by Harry Hall's frame builder of the day to help us out for a few frames that were made for the U.S. That's why the H is imprinted underneath. So yes it was built by another frame builder (and a very good one) for us, but it is one of ours.

From the stamping I would surmise this is the 112th frame provided by Harry Hall for Rourke Cycles, but I still don't know what year. There is a diamond with what appears to be "03" inside, so perhaps 1983 for year of manufacture.

Just happy to know it's genuine and not a knock-off (and rarer than I thought.) Now to research Harry Hall...