thanks - it doesn't appear to be identical to the RB cut design Keith mentions, so looks like we may have to discount that particular pattern No., though it's obviously from R.B., and doubtless from that period. Lenses and mitres were very common 1930 - 1950, and the books obviously can't include all of them.
Under the heading of S. & W., Leslie Jackson comments that one of the depositories of R.B. archives is held at Broadfield House - might be worth a punt to ask them if they have any knowledge of this design.
Anyway, it's a good piece - but then I'm a v. big fan of clear cut glass from the inter war years.