Bob
If I'm not too late......
On the Bristol Port Company website in the Forward Movements section there is a column A-PO, this is the arriving ship pilot order time. Ships generally take 1.5 to 2 hours to get from the pilot station (at Breaksea) to Battery Point. If you look also in the In-Dock section then again look for PO Time, a ship will take between 35 mins to an hour to leave from the lock into the estuary.
Whilst Battery Point is a good viewing spot, I prefer the Royal Inn (Pier Road, Portishead) which overlooks both Avonmouth and Portbury. Beer and food are very good :-)
John (Deputy Haven Master)