I realise this is extremely vague, and I have no photo so I apologise now,BUT:
Yesterday, at Par Beach, Cornwall, UK, I saw large larus that I first identified as a sub-adult marinus. However a couple of things struck me as odd;
1) the mantle was not as dark black as i would expect on marinus it was more slatey grey like a LBB.
2) it displayed an obvious ashy-grey head and neck collar forming an neat border around the throat.
It had a dark bill spot consistent with subadult marinus, and I saw it in flight as well as on the water. Whilst at the time I did not think to check the primary pattern, which I know would have been a clincher, the bird certainly had pink legs.
Any suggestions welcome?! Am fairly sure it was a marinus-size and structure would fit, but the plumage details above certainly stuck out as 'unusual'.