Hi there
@Buz TC and thank you for your question about Great Northern Divers. I believe full adult plumage birds are seen regularly here on passage. The status of the birds here in the islands is fairly common on passage/in winter (Rabbitts 2016). The nearest regular breeding population of Great Northern Divers is in Iceland (Wernham et al 2002). Some of the birds seen here on passage may also be heading to Greenland and possibly Canada. Small numbers of non-breeding birds (often immature birds) remain in summer. Breeding has occurred in Scotland on a very small number of occasions according to Hunter 1970, Hunter & Dennis 1972 and Batten et al 1990. Though not as far as I know here on the islands. Hope this helps!
References:-
Rabbitts, B. (ed). 2016.
Outer Hebrides Bird Report 2014-16. Outer Hebrides Birds.
Wernham, C. V., Toms, M. p>, Matchant J. H., Clark, J.A., Siriwardena, G. M. & Baillie, S. R. (eds). 2002.
The Migration Atlas: movements of the birds of Britain and Ireland. T. & A. D. Poyser, London.
Hunter, E.N. 1970. Great Northern Diver breeding in Scotland.
Scottish Birds 6: 195
Hunter E. N. & R. H. Dennis. 1972. Hybrid Great Northern Diver x Black-throated Diver in Wester Ross .
Scottish Birds 7: 89-91
Batten, L. A., C. J. Bibby, P. Clement, G. D. Elliot & R. F. Porter. 1990.
Red Data Birds in Britain. T. & A. D. Poyser, London