So I took it home and unwrapped it. The first thing I noticed was that it was made from 'soda' rather than 'lead' glass - listen to the rings, and you will hear the difference. What had attracted me to it was the type of 'domed' foot - very unusual for a glass of this type - more common for an earlier glass - one made in the early rather than mid 18th century. Ironically, soda glass was a much more common material than lead for early glass. It is also the preferred material for cheap, Victorian or Edwardian copies too. I have had little gin glasses made from soda which I thought were almost brand-new, but they turned out to be from around 1700. Pretty much ALL of these types of glass, however, were made after a man called Ravenscroft developed 'English Lead Glass' in the late 17th century, so a soda one in this style (1750 body with a 1710 foot) is something to be feared. Apart from the sound of it, it also shows up as bright green under ultra violet light, but I need not have tested this one with that.
The domed foot would have been uncommon, that is, unless it was a crude copy of a 'tear-stem' glass (so named because of the bubble in the stem), that has been made between 1900 and 1920... which this glass was.
So I had my fears confirmed by a real expert tonight, and tomorrow, it will be making it's way back to the seller, who - thankfully - has already refunded me with the £110 I paid for it. She refunded me because of the late delivery. There is a heck of a lot to learn in this glass business, and I may not have the time left to learn it all.