IMHO not thistles - at a quick glance I thought they looked a tad like RB 'Fuchsia', but I'm sure they're not. Unmarked cut glass, generally, is v. difficult to attribute unless the motif or whatever is very specific, but as you're only transiently stumped, then hopefully by morning you will have remembered the answer
I thought the mitred leaves had a Stuart look, but that's a guess too, and whilst the cutting is very floral, don't think we can really say art nouveau on that basis alone, though the cut pattern is attractive.
Is the rim EPNS? ...... would have thought unlikely to be Sterling, but seems there's a ridge on the glass on which the rim sits, so this might suggest they left the factory together do you think? Presumably no other marks on the metalwork.
Again, just my opinion, but would have thought very unlikely this is earlier than 1950. Apart from the fact that there has never been a book devoted solely to cut glass, there is masses of cut material around, so information spread very widely. I'd suggest the cutting has a mid C20 British look, rather than Scandi, but who knows - the usual names such as Stuart, the Webb's, Harbridge, Tudor would all be worth pursuing, though at the end of the day it may still come to nothing. You don't mention wear - is there any? Sorry this is unhelpful Adrian, though others here may have far more useful information for you - perhaps you suggest to your friend that she collects only glass with a backstamp
Thank you Paul!
Interestingly something about it was making me think Stuart too, but I'm definitely no expert on cut glass.
FWIW I knew a positive ID is a long shot on a piece like this, but obviously if we could rule out a certain region and/or time frame it would make researching it a lot easier. I was secretly hoping someone would come along and go "Ahh yes, that's the Stuart Frogs and Princes pattern"

Good point about the rim being cut in, I hadn't noticed that - you're right, it suggests it was probably made that way, rather than modified.
Apparently no identifying marks on any of it, the collar is apparently completely without any form of markings, EPNS or otherwise. The darkness of it made me think pewter, but you're right it could easily be plate, I think silver would be unlikely without any marks though. Life would be a lot easier if I could handle it!
I'm sure I will come up with a positive ID for it; that sort of thing usually happens 10 seconds before I fall asleep. I then wake up in the morning with a recollection of identifying it, but with absolutely no recollection of the ID itself

I was thinking it was older than mid-century for two reasons, firstly the quality of the cutting is better than most modern work, and secondly apparently there is some wear. The mounting doesn't seem modern to me either?
I also think there might be some minor tint to the glass, but it's hard to tell - the WB may be a bit off due to the chartreuse tablecloth...