my sincere thanks to both of you - and in view of Patricia's comments would appear that this one may possibly have been made just a little before 1850 - which I must admit does surprise me - I really had it down for the 1930's or 40's. In fact the guy at the boot sale this morning had two the same, and as I felt sure the date was well into the C20, I only bought the one.

I will try again next week and fingers crossed he may still have the other one.
Yes, I now see the meaning of the base letters - but without the book would never have know this, so maybe time to get a copy.
I don't collect these things at all seriously, but do have a dozen or so, and pick up the odd example if it looks interesting. I think there are several C19 pieces, and must admit the amethyst and blue colours are attractive.
Many thanks again.