When I lived at Wallog in 1983/84 I was astonished to hear manxies around
the house (the stable cottage, not the mansion!) from late April onwards. They
visited all summer, and if I left my window open they would sometimes wake me up
as they flew round the house at night!
I had a good look round for signs of occupied burrows on the ground, and
although there was a rabbit warren on the hill nearby there were no bird
droppings at the entrance to the burrows; it was concluded that they were't
actually nesting.
Jerry Moore
Mark Spirito's email regarding possible mainland breeding manxies had me trying to find out as much as I could and Jeremy Moore's account of birds at Wallog is documented in Birds in Wales, Lovegrove R, Williams G & Williams I, 1994.
Good to get the above from Jerry to add some detail.