I have no experience of Greenland White-fronts but would maybe expect them to be a little darker (on breast and mid belly area) with orange/ yellow bills. BWP states some albifrons can appear as dark as flavirostris Greenland race but are only separable by bill colour ie. Greenland's have orange/ yellow bills rather than pink! The 5 White- fronts ALL had pink bills, so not exceptable to me as they walked behind the Canada's and walked south at the back of the field.
A later visit to North Beach, Gunton Beach, Ness Point and Hamilton Dock I again failed to locate the elusive Iceland Gull and just saw an adult winter Mediterranean Gull flying north.