Photographic evidence supports identification as SLATY BACKED GULL

I have now had the opportunity to study some nice flight images of the Rainham dark-backed gull and from what I can see the patterning is spot on for a near-adult SLATY-BACKED GULL. It also appears that this species can have leg length in the region of cachinnans (particularly males) and that the dark grey shading is very variable, ranging from the grey of this bird to almost fuscus black

Just to remind you all, park only at the RSPB car park at Rainham Marsh reserve (Lee G R Evans)