Shawbury/Sleap error is made worse because the runway layouts are very similar. Valley and Caernarfon are not actualy that close to each other being separated by the Menai Strait and about 15 miles of land. There is also Mona between the two. It would seem to be an odd naviagational error!
I did my PPL at Caernarfon so I guess I'm more familiar with the area than this unfortunate guy was.
B