Cochylis nana would appear to be a scarce moth in Hertfordshire, although in Bucks this doesn't seem to be the case. I've seen it once before, in 2011, but have encountered it twice in two days this week: Once in Herts, whilst using a sweep-net and once in Bucks in a light-trap in my back garden. Andrew Wood in Herts reports that he has caught it for the first time just this week.
It might be interesting to see if there is a noticeable increase in the normal numbers in Bucks this year.
As the name implies, this is quite a small moth and it could easily be missed.