There was an article in the latest edition of British Birds about overwintering Lesser Whitethroats. My paper copy went out with the recycling, but if I remember correctly, it was concluded that such birds are likely to be from a race found very far to the east. There were probably photos which could be compared with the Llechryd bird.
At work today I bumped into Wendy Earle who lives adjoining Coed y Foel woodland trust reserve near Llandysul. She told me that she helps maintain and monitor the boxes which are now going up there a lot more and that there were about 20 pairs of Pied Flycatchers last spring.