None of the links worked for me Karl so I looked up your Flickr site

One of the in-flight shots seems to portray the true bill/leg colour best for me (judging other known colour hues of the species), as a paler orange colour (the swimming shot looks heavily lightened, judging by the colour of the primaries at least). Either way, its quite normal for Black-headed Gulls, particularly of this bird's first winter age, to have paler bare part (bill and legs) colouration and everything else about this bird is spot on for that species too.

Slender-billed Gulls are a very different looking bird indeed, especially around the shape of the head, neck and bill. Plus of course they have a pale eye (iris). It's always better considering the commoner options first Karl than the extremely rare ones but it rightly also always pays to ask first too if you're not sure.

Oh, and you might want to double check the identities of your Tree Pipit and female Ring Ouzel photos

May I ask what makes you think this bird might be a Slender-billed Gull Karl? What features of it bring you to that identification and more importantly I suppose, why is it not a Black-headed Gull?