A mystery bird

Spotted outside the local shop. Quite tame. The size of a black headed gull rather than a herring gull. You might say 'seagull' but I can't find an accurate match either online or in my bird book. Its grey beak and grey legs are odd. If you have any ideas, fire away, but not via an app which could be misleading. My current bet is that its a 'common gull' which oddly is much less seen here than herring gulls.