Its a slime mould, apparently.

Bill Bryson has a nice section on it. page 374 and 375 of 'A Short History of Nearly Everything'.

No idea what type it is though.

This is apparently the species, a slime mould, and the description fits well, except we must have come across it before.  

There are lots of other websites. Peter Smith at the City Council came up with it.

 http://www.geograph.org.uk/photo/914784

(Of course we knew all along it's a slime mould, we just wondered whether you did!)