Check your evergreens

It's also worth having a close look at any Holm Oak as there are several species whose mines can be found at this time of year.

Here in East Kent I have recently found:

Stigmella suberivora - the leaf around the mine quite often dies and turns brown (visible in the picture above), making the mines quite obvious, holding up to the light will confirm presence of a mine.

Ectoedemia heringella - often many mines in each leaf, with affected trees visible from some distance due to the sheer number of mines; I have seen literally thousands of mines on a single tree in Hyde Park, London.  This one is certainly worth watching out for as, since it was discovered in London in 2002, it is known to be spreading (albeit not quite as quickly as Cameraria ohridella).