This is the many-headed slime mould, Muciturbo crustacea. It is a plasmodial slime mould and consists of a single membrane containing many millions of nuclei - essentially a giant single-celled organism. It spends most of its life as a protoplasmic mass underground engulfing bacteria, decaying plant matter and other tiny food particles. When food runs short the plasmodium will migrate to the surface and differentiate into its sporing stage. It emerges as a jelly-like plasmodial goo that develops into a more complex multi-branched form bearing spore-producing structures. The various stages of development can be seen in the pictures below - the largest example is 12-15 cm across. Many thanks to Phil & Denise for showing me this fascinating organism and allowing me to take and post the photos.