Peter,
I have been trying to gather information on a flint, English Sporting Rifle for a while now. If you do a search on this site under english sporting rifle, and are patient, there is information to be had there. More specifically,
'Alexander Henry Rifle', TOF and Snyder posted a couple of beauty's, they are cap locks but they should be at the very least, inspiring. They were, are, for me.
Buy both English rifle plans from TOTW, they give you a good Idea of the geometry and what you might be able to get away with. I also have the "Mortimer" book. There is not a lot in there, but there are a few pictures for inspiration. I am going to attempt to replicate one of the locks shown in that book, lots of re-shaping and welding required, I can't wait to get to it! I have spent A lot of my spare time trying to find information on the "web", but I think it takes someone with more computer savvy ( that would be just about every body) than I to find very much. I keep being reminded of an old columnist, that wrote a piece now and then called, "Things I learned while looking for other things". The search alone is worth the effort.
In the mean time please share what you find, and I will as well. Good luck!
Robby