Surely a pilot would have realised by looking at his map /outside that he was passing the menai strait
He was meant to be! He was diverting to EGOS. Sleap and Shawbury ATZs are only about one mile apart, the runways have the same orientation (if I remember correctly), Sleap lies partly within the Shawbury CMATZ and when landing at Sleap you have to talk to Shawbury first so it's probably a much easier mistake to make than some.