On our way back from Battersea Park during the London Open House 2011 I saw what had to be a fairy ring. Closer inspection revealed the fungi responsible.
Fairy ring in Battersea park 1
According to wikipedia they can grow to be several hundred years old. http://en.wikipedia.org/wiki/Fairy_ring
Fairy ring in Battersea park 2
Easy to spot if you know what they are. We saw several more last weekend on our way through King George's Park when we went for a wander down the Wandle to Merton Abbey Mills. Isn't nature wonderful!