The best source of information I know of is Leicester University - http://www2.le.ac.uk/departments/biology/people/bailey/res
Gives useful information on the all-female clone of the species, the less common Giant Knotweed that is hermaphrodite but can't set seed without cross-pollination, and how to tell these two and their hybrid apart.
The photos are largely unlabelled - right click on them and select 'View Image Info' to see the associated text, which will tell you about it.
I've come across a thorough paper on Giant Hogweed, from Countryside Management Publications, including identification, dangers and removal - by both chemical and physical means. It came up in a search and I don't know the URL, so I've filed it here. I trust it will serve your purposes, but as I didn't write it can't vouch for any mishaps using it.