Dating simplified.
If it has a wooden handle and a serial number up to 50,000 over the trigger it was probably made between 1924-1935, if it has plastic handles without the Webley Logo on and a serial number 50,000 - 65,000 above the trigger then it was made between 1935 and 1939. If it has Plastic Grips with Webley Logo and Birmingham 4 marked on it 1946 - 1958, if it just has the same grips and just Birmingham marked then it was made 1958 - 1964.
Btw post war had knurled barrels and serial numbering ceased, the numbers on post war Mk 1's were batch numbers.
Values according to condition.
Regards
Robin.