Walking along the towpath of the Grand Union Canal (somewhere SW of Heath and Reach) today I spied the accompanying hedgerow had wild Hop in it and, in one area, there were leaf-mines that looked like this:
This is caused by the caterpillar of Cosmopterix zieglerella which is a relatively scarce and local moth. Indeed, although there was Hop off and on in quite a long stretch of the hedge, leaf-mines were only evident in a short portion.
It would be nice to post a picture of the adult - a very colourful moth.
It might also be worth pointing out it was very close to the border with Bedfordshire.