Paul et al - The reason that the registration wasn't in any of the published lists is that the registration is under Class 4 - Earthenware, not Class 3 - Glass. Presumably Scotney & Earnshaw were keeping their options open should they have wished to have it made in pottery or china. Please ignore the old popular "error" explanation, as it happens too frequently.
The punch font is quite distinctive, particularly the RD, so it might not be too difficult to identify the mouldmaker.
Paul, you will find a natty basic watermarker from a highly respected source here:
http://zippyhelp.com/imagetools/watermark/watermarker.htm
Here's one I did earlier (click image to enlarge):
(http://glassgallery.yobunny.org.uk/albums/userpics/10318/normal_DSCF1298a.jpg) (http://glassgallery.yobunny.org.uk/albums/userpics/10318/DSCF1298a.jpg)
The Cock Hotel, Stony Stratford, Christmas 2011
This, along with The Bull, a few yards to the right, is the most famous Inn in the English speaking world.
Image max dimension (width?) must be 400, 500, 640, 800 or 1000 - I used 800. Size set at 10. Intensity set at 3. I did it twice, once for each line of the watermark.
Bernard C. 8)
ps - I've just noticed that I copied Frank's US spelling of licence - too late now! ;D