This plant was carpeting a large area of dried up pond margin. The whole area (several metres) was covered by it. The flowers are very small (smaller than stonecrop for instance). Flowering 25th August 2011 in Leicestershire.
It does look very like a stonecrop (Sedum or Crassula possibly?) Except that the flowers have four petals instead of five. According to my book there are a number of species, and some have smaller flowers than the familiar stonecrop.