Professor of Listening
I agree with Mark's analysis. Sooty Gull should look pretty dark (almost blackish) on the underwing and in some of the photos it's much paler. There is white on the lower rump/ uppertail but this seems too narrow for Sooty Gull. LTS often shows an extensively white band with barring in this area and I think the quality of the photo just means the barring is obscured. The white shaft on the outer primaries also seems to appear quite consistently, which is good for LTS.