Essex County Council operate a flood warning network of 36 sirens. This document on their web site gives plenty of information on the system including how the sirens are activated and their locations:
http://www.essexcc.gov.uk/vip8/ecc/ECCW ... elOid=null
Over the course of a few trips, my brother and I visited all but one of the sirens and I've uploaded lots of the pictures here:
http://s92.photobucket.com/albums/l28/A ... ?start=all
The only one not pictured is on an island owned by the Army (Foulness Island) and I think it's pretty unlikely I'll ever be able to get a picture of it.