I can confirm that this will be my last post about Yellow-legged Gulls. Well, at least for the next two weeks! I photographed this 2nd-summer bird at Rotherhithe on 7th July. It had me, and others that I've subsequently emailed, perplexed as it's not the most obvious of Yellow-legged Gulls - and indeed shows some characters you wouldn't normally associate with the species.
At the time I thought that it was potentially a Yellow-legged Gull based largely on mantle colour but obviously it has a fully dark eye, tepid bill colour yet the state of moult isn't ideal but it seems to show no white mirror on P10. There certainly seem to be too many features wrong for it being a cachinnans (which it isn't) - axilliary pattern and colour, tail pattern, legs and perhaps you’d ideally want some grey inner greater-coverts. If you ignore all the bare parts and eye colour you could say that there isn’t an awful lot wrong for michahellis. However, I'm not fully convinced it is a pure Yellow-legged Gull I wonder whether it could potentially be a hybrid... perhaps something like a herring x mich or even a mich x cach but that is just pure speculation.
Chris Gibbins added that the bird shows some mixed signals too, but pure birds are so variable anyway and in 2nd-summer plumage there are no ways to identify hybrids. Interestingly he also suggests that if it is a pure Yellow-legged Gull it may be sick and hence the dull/delayed development of bare part tones.
Thanks to Chris G, Stu P, Alan C, Josh J and Andy L for their comments.