no, peter, not gavia immer has a rosy coloured bill but podiceps cristatus
i've just checked some images and i think i'll sway back to great northern diver/common loon: it's not so much the thickness of bill but more the shape: black-throated has a more parallel edged bill and a curved culmen, in GND it generally
looks more triangular, like in the OB. as for size, some small GNDs can be as small as BTD. another thing that might count (but i haven't found it as separation feature) is the feathering reaching to the middle of the nostril in the OB, as in most GND pics, while in BTD in most it doesn't reach the middle of nostril. the nostril itself tends to be more split in GND than in BTD - see the OB pic, so this might be enough to sustain my initial feeling of it being a
gavia immer.