I live in the area you mention. 
‘My’ swallows have fallen from 3 pairs 3 years ago, 1 lone male for 10 days 2 years ago, 1 male for 1 day last year and 1 male for 2 separate days this year and now gone.  A neighbour’s have fallen from 6 pairs to 2.
Is this replicated across the county?

Talking to two farmers on the southern slopes of the Mendips between Cheddar and Wells yesterday, both told me that “their” swallows had not arrived this year.  One reported that this is the first year that he has had none by this time and he’s in his 70s.  Normally, he told me, you can rely on them to arrive within a day or two of 9 April, though last year they were a little later (the 13th, I think he said).  Has anyone else heard or know of similar reports?

Apologies if I’ve posted this in the wrong forum - please feel free to relocate it - but I thought it would receive more views and might stimulate more discussion here.