Confidence: I'm as sure as I can be.
Notes: Available in a variety of wing-lengths: all the ones in your picture have the shortest form. Foodplants are grasses. The behaviour you show is typical. It seems the name derives from a a relative, the Chinchbug, which is an important cereal pest in North Americal. Our species is harmless. A very similar species occurs in this country, but only at Folkestone Warren.