Hi Paul,
Quite right. The main feature is the dark inner primaries. No hint of pale webs at all so basically you have an entirely dark trailing edge to the wings.
The tail looks thin and the black band extends a long way up the tail (outer retrices show a thinner band on Yellow Legged Gull).
Subjectively, the bird looks slightly greyish and the head is dirty looking with a weak, droopy bill. Yellow-legged juvs are often slightly more strikingly black and white with whiter heads and thick, lozenge shaped bills.
Lou will correct the above if necessary.
Cheers,
Andy.