What a chalk stream does to a valley

There are only a couple of hundred chalk streams in the world, and nearly all of them rise in southern England. The Lambourn is ours. Rain falls on the downs, sinks through sixty metres of chalk, and emerges years later cold, clear and steady — the same few degrees in August as in January.

That steadiness shapes everything in the valley: the watercress that once left here by train, the racehorses on the gallops at first light, and now our growing rooms, which borrow the valley's patience. Slow, cool and unhurried is how the Lambourn does things. It suits mushrooms perfectly.