A yellow slime mould, probably Fuligo septica. During the spring, foliage had grown up over the places I had seen slime moulds during the winter, but this specimen was growing on recently mown grass, just a day after the first heavy rain following a month of drought.
Another day later and it looked like this:
But about 20cm to one side the following had appeared (I am fairly sure it wasn't there the day before):
It seems that slime moulds can change both their appearance and their position significantly in just 24 hours.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-07-24 and 2010-07-25.