Following Sarah's post below I looked at the recent county records for Grey shoulder-knot to compare spring and autumn records. The chart below shows those records from 2001 to 2015.
In 1984 Adrian Fowles stated that this moth was "Formerly not uncommon in Ceredigion's oakwoods, this species has undoubtedly declined and just eight specimen have been recorded in the last thirty years". It seems to have had a bit of a recovery since 2000 with a peak in 2010. I wonder if the difference between spring and autumn numbers in a year, are affected by the summer weather, eg. 2006 which was a good summer. Do the difference between autumn numbers and spring the following year, reflect on the winter weather?