Around here all the horse chestnuts took on an autumn colour in about August - you could tell which trees in a hedgerow were chestnuts as they were the only ones which were bright yellow. It was the same story last year. If you look more closely at the leaves you can see the colouring isn't anything to do with autumn at all.
This is chestnut leaf miner damage. It's caused by the larva of a moth which has become absolutely rampant in the south-east of England. It comes from southern Europe, and until recently was minding its own business over there, but in 2002 the first ones crossed the water and turned up here. I believe it's now making its way north.
The Forestry Commission are keeping an eye on it, and they've asked people to let them know if it appears in places where it hasn't yet been seen (a few more dots are ominously appearing on their map each year). Apparently, dramatic though it looks, it doesn't do any damage to the tree, though if it appears at the same time as a nasty bark disease called bleeding canker the combination can be fatal.
It seems to me, though, that if you completely defoliate a tree every year for a number of years, it can't do it much good in the long run. I love horse chestnuts - like most people I played conkers as a kid, and I love the fact that they're so big and strong and sort of ancient English forest-y. They're the sort of trees you use as landmarks, the sort of trees you rely on for your sense of identity and place. So to see them all looking so sick, so early in the year, makes me fear for their future. It feels all wrong, like daffodils in December. If this is global warming, you can keep it.