I came across this individual on Valentine's Day, a 1st-winter (2CY) that was sat tight on the ground for a long period before it showed a little better; to be honest first impressions on the ground had me thinking dark argentatus Herring Gull. Most 1st-winter LBBG don't appear as dark as this individual this late in the winter ( or at least those that I've seen in Spain and in literature/images don't) but after some head-scratching I think this is probably a better fit for Lesser Black-backed Gull than the alternatives. The greater coverts don't appear typical for most LBBG but other features seem to fit, black tail band, white rump and undertail coverts with large sparse dark spots; blackish-centred mainly plain tertials with thin off white edges; quite a few dark retained juvenile scapulars and the patterns on the moulted upper scapulars appear dark and more in line with LBBG; dark head, mask and acquiring a '1st-summer' pale base to the bill.
The droopy wing relaxed stance would also seem to fit with LBBG. Any comments welcome.