Rather unusual for this area as 50 miles inland on track coming into garden,not that clued up on gull's so maybe this
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Black headed gulls do tend to nest in land on islands in lakes and marshy areas so 50 miles inland is not a problem.
Thank's for the explanation, I know there not just coastal but did not think it would be that far inland, thought more like 10 miles,will watch out for them now.
Lat/Lng: 55.05705, -2.23233
OS grid ref: NY852736