One more thought Kevin,
Make certain that you know which berth your ship is leaving from, Cunard will use dock gate 4 / QEII terminal, whereas P&O/Princess & smaller lines use Dock gate 10, the smaller lines are in berth 101 / City Terminal, whereas the larger ships use berth 105 / Mayflower Terminal.
If multiple ships are in though, the berths often change.
Closer to the time, checkout http://www.southamptonvts.co.uk/cruise_ship.asp
it lists all the provisional cruise line movements & is 99% correct.#
Hope this helps, and have a fantastic time.
Alan & Katrina