Longbranch, you lucky dog, you have a fine shotgun in your possession. While an economical gun, it will embody the balance and handling characteristics which make English guns so desirable.
Tiriaq's information is correct, but I can add that your gun is almost certainly made for Army & Navy CSL by the firm of Webley & Scott, and is most probably based around the Model 500 action. The give - away is the patented "screw grip" system for the top lever, which incorporates a third fastener using the rib extension. In addition to the fastener, your example includes a doll's head, something I have not seen a lot of on screw grip actions. Another nice feature is the bushed firing pins.
It would be very useful to see photos of the water table and barrel flats. It is very likely that the gun has 2 1/2 inch chambers (although original 2 3/4 chambers are possible but rare). I am guessing that the gun dates to the late 1920's or early 1930's. It would also be interesting to know the length of pull and the barrel length. The chokes will also be worth paying attention to - many of these guns were choked full and improved cylinder, an odd combo these days but very useful for an all around shooter.
As to the value, assuming no mechanical issues and no dings in the barrels, guns such as this sell in the $1,750 range (plus or minus $250 depending on wood quality, length of pull and condition). If the chambers have been lengthened by a gunsmith subtract $300. 28 or 30 inch barrels are more desirable than 26 inch.
These guns are absolutely wonderful for use on grouse or pheasant. While they can be a bit of a pain to be made to fit you (you might get lucky), once you have experienced the effortless manner in which they mount, swing and shoot you will be spoiled for anything else.
Like all British game guns they are not made for heavy loads. Kent distributes the GameBore Pure Gold shells which were made specifically for guns such as this. They work like magic on the game they were intended for. Find a flat of number 6's (English measure, more like #7's here) and go hunting!
Congratulations on your acquisition, please make the best of it.
Sharptail