Returning via Calais

David2115

On our return in a 10 days we thought we would not book a site for last night and just pitch up at the port for early ferry the next day, now I'm thinking that if not too tired from drive we might just get an earlier ferry to the one booked, do u think P&O
will allow this free of charge, or charge difference in 2 fares or charge a completely new fare. Thankyou in anticipation of replies.