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There are a number of features in this photo which point to GND rather than BTD.
The large bulky angular head is indicative of GND rather than BTD which has a much smoother head profile. The dark patch at the base of the neck is also diagnostic of GND (or White-billed!). The paleish back also points to GND rather than BTD which at this time of year has a plainer back. I think the white flank patch is on this occasion deceptive.
At this time of year GNDs are the most common diver off the west coast of Scotland.
Hope this helps
Lat/Lng: 57.222082933351, -5.6745457649231
OS grid ref: NG782205