Hoody in the park

I've watched Carrion Crows fly low across the Round Pond in Kensington Gardens and playfully dive-bomb the gulls perched on the buoys, even though they have no intention of perching there. Other gulls are another matter and many like to be in pole position on buoy number 3, much favoured by the wintering Mediterranean Gull. The series of pictures below show the German-ringed adult, now stunningly complete in breeding plumage, suddenly becoming aware of an incoming Common Gull and it utters a call; perhaps a threat or submission call. With menacing intent, the Common Gull drops down, sinking its beak in to the Med Gull's back while attempting to gain a foot hold on the buoy. Both end up in the water, as the perpetrator slips and the submissive hoody awkwardly belly-flops into the pond. After all this, the Common Gull flies off and makes no attempt to take up its perch. What would be the real reason for this aggression? Does the Common Gull recognise the other larid as something 'different', in much the same way that petrels are often harassed? Or perhaps it is to do with food resources and the 'pecking order', the Med Gull being the nimbler gull, quicker at snatching the artificial food provided by the tourists. Thanks to Des McKenzie for the tip-off.