Some good suggestions from other bloggers, but this morning I had confirmation from Professor Lynne Boddy at Cardiff University that it is a slime mould. Slime moulds were once thought to be thought fungi, but are now known to be not even closely related. Professor Boddy suggested I contact a colleague of hers for positive identification, but I'll leave it with just the general identification. Mystery solved.
And the thaw has set in in Somerset. Noticable how many birds were singing today, makes me realise that when the temperature is below freezing continuously, birds are just concentrating on feeding.