Any Lawn Experts? - please help!

From my RHS book, it seems to be an algae / gelatinous patch: symptoms = slippery patches of green or greenish-black form on the surface.

They are commonly found on any poorly drained, poorly aerated, or compacted sites.

See here for RHS webpage on them
http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=414

Thanks for the replies and the links. The RHS one is very good. We are going to spray the sulphate and then try to aerate a bit, problem is it is shallow soil there on top of compacted hardcore so might take a bit of work. The area was a field but the previous tenant dug out all the soil and filled it with road scrapings to make a hard standing. We have tried to reinstate the grass and to most part it is great just this area but the is an oak tree so it could be the runoff from that which makes it worse.