On 2009-09-28 13:02:44 +0100, Stewart Robert Hinsley
said:

Marginal water plant, with spikes of blue flowers. I think that it's a
monocot, and perhaps an aroid. Probably cultivated (and doesn't
immediately look like anything native).

http://www.flickr.com/photos/27317581@N06/3961638427/

Thalia dealbata, perhaps?
snip

--
Sacha