As you may have guessed from the habitat, it was a Reed Bunting. On the face of it a pure albino, but some darker feathers in the secondaries and tail suggested it may have been exhibiting an extreme symptom of leucism. Fortunately it behaved and sounded like a Reed Bunting. Within a few minutes it took off and headed south. It is proof that even apparent resident species actually migrate to some degree, since I suspect we might have noticed it if it had been around all summer.
The rest of the morning passed pleasantly enough. We recorded the first four Stonechats of the autumn, had a distant view of a late Whitethroat, and scanned through the hordes of House Martins still milling about (at least 44), until I eventually spotted a single Swallow.