It seems that some I found were flies and a couple on Elder and Ivy, make tiny mines and feed inside the leaf. The only reliable way to find out what they are is to rear them.
I saw quite a lot of very small, light brown moths flying. One eventually landed on a buttercup leaf. It only stayed long enough for one photo, which is not a great one. They were Nettle Taps.
Now for the Leaf Mines, it's a phyllonorycter sp. on oak. You can clearly see it left the leaf on the underside.
Top side of leaf .
phyllonorycter sp. bottom side.
I thought I'd show the other two mines out of interest. The Ivy one is first. I'm intrigued by the reddish area near the centre. I have no idea how it was caused.
Leaf Mine on Ivy. Top.
Mine on Elder. Top.