Hi Geoff,
The number stamped under the grip is a batch number rather than a serial number, so doesn't really help with dating. There are however some subtle differences that can help, so a couple of good pictures would be useful.
A rough guide is the presence of a B4 post code after the Webley address stamped on the side of the pistol which indicates manufacture before 1958 when Webley moved from their Weaman St premises.
Regards
Brian