There are 3 things that can do for you in water below 12 degrees. One is sudden immersion syndrome, where you gasp with the shock, and drown. Another is freezing cold injury, which involves losing your extremities to the cold. And the third is hypothermia. At Hampstead Ponds, for most of the winter the water stays around 3 degrees. Sometimes the edges will freeze. Despite this, or maybe because of this, the pond attracts a small band of dedicated cold water swimmers.

"February is the real killer", says Tony, a Londoner and veteran of several winters. The cold doesn't seem to do him any serious harm, though he swims for longer in summer than he does in winter. "4 degrees today? One degree warmer. You won't notice the difference. It's still bloody cold. I must be mad."

London, January and Febuary 2006