Hmm, later in the day I threw out a nice big juicy mackerel as I bought it on Thursday then bought a Chinese takeaway instead, then didn't eat it yesterday either. That is still there too, perhaps they only eat earlier in the day.

Actually if it wasn't for a real bully more might have been eaten earlier, there was one which was not the same type of gull to the rest, he was a big bully and chased the other gulls away. I am not sure what he was, at first I thought it was a juvenile herring gull but it was larger than all the adults, then I noticed it had a black beak instead of a light one. I must try to find out what it was.
Edited to add that it probably was a juvenile herring gull, I just found
this photo which also shows a dark beak.