Worryingly, I've found a lot of failed breeding sites, where 1st or 2nd instar larvae had been feeding. Most of these have been checked twice, just in case of oversight. It looks as though the incidence of 1st and / or 2nd instar fatality has been unusually high this year. This may be because of the cold, wet August, though I suspect it may be due to the leaves being too thick and coarse following an early and rapid spring.
Here's a trio of leaves bearing seat pads and feeding marks of lost young larvae (+ egg case bases) -
Note the leaf tip damage.
I rarely find any on narrow-leaved sallows in / around Savernake. Here's an exception - and a rare example of a larva not on a leaf tip -