Big Bird

We found this young bird sitting in our garden and wondered what it was, he was just strolling around, at one point it was sat on the water of our fish pond, how it got there we have no idea a couple of times he flapped its wings but didn't seem able to fly,it was also worrying our cats so much they were climbing the glass of our conservatory trying to get at it. i found out later it was a juvenile gull but in our garden it looked huge.
Our gate was open and after about a half hour he strolled out and was walking up the road.