In reply to Flamingsarah:
I got that flight last year and was in plenty of time for the last ferry. In fact, if I'd got a bit of a wiggle on I might have made the ferry before it, which left just after I arrived at the dock. I ended up having time for a beer and a pizza while I waited for the next one.
The Gatwick flight was late in that evening and the ferry waited for them so chill out and don't worry.