Thanks for all the replies. If I want to travel on the RedJet passenger ferry (which I do) it sounds as if I must travel after 0915 i.e. on the 0945.
I'm not so sure. Have you carried out my suggestion of writing to the company along the lines that I recommended above?
Since this will mean I don't get to Southampton Central train station until approx 1030, I think the Network Railcard restriction becomes irrelevant.
Quite possibly, but it is unclear whether the restriction is intended to apply to rail only, or whether it is intended to reply to ferries. I doubt the ferry company would be aware of such a restriction, to be honest, but I'd check.
I am however more generally intrigued as to whether the Network Railcard time restriction is on trains departing before 1000, or on trains arriving at London Terminals before 1000, as suggested above.
The Network Railcard restriction is on departures from 1000. This over-rides the ticket restriction, which is based on the arrival time.
In other words, the ticket itself (ignoring the Railcard discount) would be valid much earlier, it is only because it is Network Railcard discounted that you cannot catch a train until 1000. However the ferry company is unlikely to be aware of or care about the railcard restriction which is probably not intended to apply to ferries (there are easements on Island Line allowing trains quite early to connect with ferries that connect with trains after 1000!)