Seeing Ian's & James's comments, I thought I'd do a bit of investigation into my own records for this species, looking at the two broods to see if there was a pattern. I think there may be, but it will need someone else to confirm my ideas.
I started trapping here in East Lydford in August 2005, so 1st brood figures are not available for that year, but there were 26 records totalling 40 individuals up to the end of August. The following figures show some interesting aspects.
2006 1st brood 19 records 57 individuals
2nd brood 32 records 128 individuals (hot dry Summer)
2007 1st brood 10 records 19 individuals
2nd brood 6 records 8 individuals (cool wet Summer)
2008 1st brood 11 records 30 individuals
2nd brood 12 records 40 individuals (average Summer)
2009 1st brood 17 records 58 individuals
2nd brood 11 records 50 individuals (cool Summer)
2010 1st brood 12 records 47 individuals
2nd brood 1 record 2 individuals (cool Summer)
2011 1st brood 5 records 10 individuals
2nd brood 1 record 1 individual (cool Summer)
It seems to me that the succession of cool Summers in recent years has had a cumulative adverse effect on this species. Whether this is merely speculation on my part, and whether other species have experienced the same levels of decline is open to debate. I remember that the cold & wet Summer of 2007 had a disastrous effect on numbers of Lunar Underwings in particular after record numbers the previous year. (From 2/300 per night to barely double figures). Food for thought.