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There does not appear to be a yellow tip to the bill and the legs look shorter than the Sandwich Terns which led me to ID this as a possible 1st summer Arctic
The tern in question has a blocky head and long bill (for a juv). The yellow tip is an adult feature. An arctic tern would appear to be sat on its feet and the head would be small and rounded. The bill would be shorter and spikier
Can you get any more detail on the rings on this bird? - it is ringed on both legs, which made me consider a 1Y Roseate Tern. The clincher would be the size when viewed in the field, but overall the the length of the legs and bill do seem to point to Sandwich.
If the original images allow the ring details to be established then you may even be able to trace which colony it was ringed at
Sorry I have looked at the originals and they are not any clearer.
I think that if this were a Roseate it would have been observed and logged by the number of birders that were present in the area looking for the Black Tern and Little Stint.
So it is most likely a Sandwich. Thanks to everyone for their comments and ID.
Lat/Lng: 50.7309, -1.5397
OS grid ref: SZ325924