I have to say that we use P&O as for our unit they seem to give the best crossing price, and i have nearly always had to change part of a booking and have incurred no charges other than a £10 admin charge, this last time i have even changed things before we have done the first leg about three times and have still only paid one admin fee and this included seperating the two vehicles going over to different dates from the origional same date and changing the passenger details 3 times