A juvenile bird, quite mobile but giving good views at north end of the reservoir this evening in with other gulls. Apparently has been around for a week or so now. Also of note were singles of kingfisher, sparrowhawk and raven.

Only had a digital camara with me so had to make do with this scope shot.

Oliver, that's a juvenile Black-headed Gull...unless I'm very much mistaken and I'll happily stand corrected if so.
I popped down last night at 9pm and spent 45mins on site and there was no sign of the Little Gull. I spend hours at SBR every week and I'm sure if a Little Gull had been present for as long as you suggest I would have seen it...although I have been known to dip birds on more than one occasion as several people who read this forum can vouch!
My reasons for suggesting this is a BH Gull are as follows:
1- the legs look too pale.
2- the amount of black on the head is not right...Little Gull should just have a spot behind the eye rather than the black going up behind the ear coverts on to the top of the head.
3- the bill is too long and is pale with a darker tip, Little Gull should be all black and much shorter.
4- finally this bird is rather brown whereas Little Gull should be greyer.
Unfortunately there is nothing in shot to compare size, but Little Gull is about 1 third smaller than a BH Gull.
Of the three juv BH Gulls at SBR last night one was noticeably darker and I am certain was the bird you have photographed. It is not that unusual to find "browner" juv BH Gulls, see Gulls of Europe, Asia and North America by Olsen and Larsson.

Though I don't have any photos to compare size, the bird was significantly smaller than the black-headeds it was with, and in flight showed quite distinctive black V patterns on the wings and flew in the little gulls typical bouyant, tern-like fashion. However, I was made a little suspisious of the slightly paler base to the bill, but presumed it was just variation as the other id features seemed stacked towards little gull, though you may be right.