Mikey said:
Also I realised that the second link which should have linked to a
close up of the seedhead, was just a copy of the first...doh!

Should have been ...

http://static.flickr.com/30/44748610_4a0a9334dc_o.jpg

Comparing these with pictures of yellow or purple nutsedge Cyperus
Esculentus or Cyperus Rotundus reveal that the seedheads don't quite
match up...

Any more ideas?

I'm pretty sure it's a Cyperus spp. (based on my identificaton guide).

But one website I cruised through said, "The genus Cyperus is immediately
recognizable, but determination to species often requires minute dissection
to examine the mature achenes."
--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)