Hi Clare,
You've inspired us to book our own Harwich Seal trip in a couple of weeks time and we also found a good trip on the Blackwater out of Maldon. It's a 12hr barge trip with Top Sail Charters, 7am to 7pm on October 14th, so the start and end of the trip includes sunrise and sunset. It also has someone from Swallow birding on board for bird ID.
Best Wishes,
Trevor