Seriously...I have seen small flocks of BTD's like this but never GND's and I've just flicked thru Jonsson's beautiful
'LOMMAR' and they look like the BTD'S in that wonderful, but all too brief, swedish book.
I'm apt to agree with TWM (not just because we have nearly the same initials!) - small flottillas of black-throats are quite usual whereas all of the GNs I've seen have been basically solitary (winter).
The neck markings, in my most humble opinion, aren't diagnostic (I refer to sketches made of both species over the past couple of months - each species shows some similarity, and often it's how light hits the neck etc).
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The near bird is doing some contortion which I can't quite fathom, but it's possibly the top of the head we can see.
If asked to put my house on it - I'd be moving into a caravan. Personally, however, I think that we have a group of the same species - I feel sure that we'd notice a difference in scale were there GN & B-ts here.
However - I'm nearly always wrong so we must take that into account. Happy Diving.