Lee Evans kindly followed up on the gull sp (see below) and contacted gull master Peter Adriaens. It was good to hear from Peter again, who guided me on the identification of the Short-billed Gull on the Azores from 2003.
He drew my attention to these links from another gull expert, Visa Rauste: http://www.helsinki.fi/~rauste/gulls/hannu11.html and http://www.helsinki.fi/~rauste/gulls/parnu02.html
Peter says that the situation is complicated by eastern baltic argentatus herring gulls that can show features which recall Caspian. To make things even more interesting there is genetic evidence provided by Pierre-Andre Crochet et al that this variation of the eastern baltic argentatus is due to the presence of genetic material from an unknown perhaps even ancient gull species.
It really dont get much better than that in gull identification- a 'hybrid' with an extinct species.