Hi Steve,
I'd date your rifle as mid to late 1938. Can't see the keyway on the barrel but as was suggested on Craig's thread, look for a slight step where the barrel locates within its housing. The bead on one of your barrels does not look original but the lower profile blade does.
Incidentally, if you include a pic of the rearsight, that may also help date the rifle. Webley changed to the turret type of rearsight, which was also used on the Mark 3 in 1938 (factory technical drawings are dated Feb 1938). Webley literature suggested the change took effect from SN 12400 but anomalies either side exist. The Intercepting Sear housing was also redesigned in October 1938 and yours looks like the earlier type. However, as with all serial production, it takes time for changes to work their way down the production line.
If you want to know more about these classic air rifles, I too would also recommend you obtain a copy of Chris Thrale's excellent work Webley Air Rifles. It will tell you all you want to know and more.
As for value, that can be very subjective. For instance, take a look at the prices being asked by JC Militaria on their website and compare them to the results of the Holts sale last week. If you visit an arms fair, you will see both high and low prices being asked.
Kind regards,
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.