You may well have noticed that many (most?) of the Horse chestnut trees around you are starting to look a bit manky. Their leaves have become covered in white and brown blotches.
Those blotches are actually leaf mines, home to the larvae of Cameraria ohridella, the Horse chestnut leaf-mining moth (the brown blobs in the mines). According to the UK moths website
This species was discovered near Macedonia in 1985, and since then has spread rapidly to other countries in Europe. It was first discovered in Britain at Wimbledon in south-west London in 2002, but possibly had arrived the previous year, as it was quite plentiful. It is thought that the species may be expanding partially due to accidental transportation by man, either by road or rail. It has now been found quite extensively in the south-east of England.
Obviously, since that website entry was written, the moths have now spread from south-east England to south Wales and, indeed, to parts much further north. You’re mostly likely to see the blotches between June and September and, though you might not like the look of them, they’re not thought to inflict any permanent damage on the tree because, of course, the leaves are shed in the autumn anyway.