Very nice and exciting to have found it yourself. I have always suspected that weapons in 'urban' waterways often came there by a need to destroy evidence. Below is a picture of a dagger with a 7 7/8 inch blade reputedly found along the Thames, though it is far less intimidating than the one you mention.
There is a nice article on these daggers at MyArmoury:
http://myarmoury.com/feature_spot_bd.html.