I think I may have found out what it is.
It goes by the charming name of......(insert drumroll)
Dog Vomit Slime Mold. (who on Earth thought of that name
)
http://botit.botany.wisc.edu/toms_fungi/mulch.html
The description seems to fit what I have but I'd be keen to hear if anyone agrees with me or has other ideas.
Cheers