Don't know the answer to that, but if not The Caravan Club have a site, Rookesbury Park that have Ferry pitches.So you could stay overnight there if there's any available.
------------- Debbie.
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I would have thought that given that you ferry is the first one of the day I doubt you would have a problem unlike Dover where ferries go far more frequently. You could always give the ferry company a ring just for peace of mind. Which route are you taking, if its M4 and then exiting onto the A34 north of Newbury there is a big service station at the interchange. Or go on a bit further and use the Sutton Scotney services just north of Winchester which tends to be less crowded but the facilities are not as good.