Contact Shooting Times magazine, based in Windsor, UK. They've got a resident expert called Geoffrey Boothroyd, who's the fount of all knowledge on antique firearms and seems to specialise in trcking down details of some of the most obscure guns on behalf of readers.
Failing that, try Sotheby's or Christie's - they've got offices worldwide which are listed on their websites, and have experts on old sporting arms.
Lancaster was on of the top London gunmakers, and the records of many of them have now been subsumed into those of thre three main survivors - William Evans (in Pall Mall), Holland & Holland (in Bruton Street) and Purdey & Sons (in, I think, South Audley Street). One of them may be able to furnish details of the original owner and could help.
As to the potential value of the rifle, it could be worth a lot, and more if it has the original case, cleaning equipment and bullet moulds.
Good luck!