In message , Bob Hobden
writes

"Stewart Robert Hinsley" wrote ...
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/

I was also going to ask you about the previous plant on my photostream,
but in the nick of time I realised that it was Hypericum humifusum - a
very different habit to other St. John's Worts.

Looks like Pontederia cordata to me.

Thanks (to all). That's it. I thought it looked vaguely familiar, but I
didn't know where to start looking.
--
Stewart Robert Hinsley