Did a quick look at Guns International, they list 13 Watson Bros. guns, the cheapest--The Cheapest is $5,000, WOW.
You have just become not a owner but a caretaker of a very well made English gun. Guns like the Watson don't come along that often and are built to a standard way above todays mass produced plastic/stamped guns.
If you want the gun serviced, DO NOT take it to the corner gun shop. A specialist is needed to work on this gun, one who works just on English guns.
Check with Connecticut Shotgun www.csmcspecials.com
they may do the work or can recomend a very good gunsmith. By the way CS has a Watson Bros doubles listed under used guns it's $12,500!!!