Hello Martyn - regret I can't help with anything positive, but can give you some thoughts
. No doubt champagne has been drunk from just about every shape of glass, but I don't see this as one of the classical shapes i.e. the flute, the saucer/coupe or the tulip, but not aware of this shape associated with the drink.
Believe originally the ale glass was used, followed by a purpose made flute, then the Regency saucer/coupe and finally a mid Victorian invention called a tulip shape, which I can't recall ever seeing. I would imagine your glass to be C20 in date.
Unfortunately, unless you can find this particular cut design in the catalogues you stand very little chance of finding an attribution - there are literally almost thousands of different cut designs, but it's certainly worth starting a search on the 'glass replacement' sites - you might get lucky. The cutting does look rather special, particularly the under-foot cutting, and the way that the leaves are given a deep cut to one side to provide contrast or light reflection or whatever, looks quality. I think Stuart were keen on that feature. Have you flicked these - are they lead glass - I'd have thought they would be.
Very nice glass, and best of luck in searching - but be prepared for a long haul.
P.S. Could someone please add the links for the two most useful replacement sites - can't lay my hands on them right now. thanks.