Most of it became a retail park, but QinetiQ ran a small area of the site for bridge test and wet gap crossing until a few years ago. It now has, or is about to become a supermarket and housing estate: http://www.bournemouthecho.co.uk/news/9320528.Multi_million_pound_plans_to_transform_derelict_Christchurch_riverside_site/
The pub/restaurant on the retail park is called The Bailey Bridge, so at least some history remains.
Andy