OK, with British Red's help, we've decided it was made between 1855 and 1875 based on the proof marks - I finally managed to get the forestock off to find them. It's not been re-proofed so it can only really be used with black powder and probably should be tested for that!
This photo shows the proof marks but also clearly shows the Damascus swirls
This photo shows the proof marks but also clearly shows the Damascus swirls