Too hasty by half young Thing– it would seem that I have been merrily led down the garden path by a sub-adult bastard Herring Gull.
Key i-d points that were missed by yours truly were traces of brown on the tertials and greater coverts and there are wide white tertial tips, which are lacking, or almost so, in RBG (this information comes to you courtesy of the expert gull bods at birdforum). It did tick some of the other RBG boxes though in my head - size, mantle, iris, bill band, movement, fluffiness etc etc but there you go. Ring billed gull, no. Ring billed bollocks from me, yes.
Well I shall not be falling into that trap again little bird. You and your like may not be straightforward, but I shall prevail!