I have just discovered that P&O's web site is innaccurate!!!! - apart from being expensive.
I got a quote on their web page for exactly the same crossings as we used this year (cost £266), and it came up with a price of £333!
I phoned their reservations line, asked again for the exact same crossings, and was given a price of £259 - £7 cheaper than this year. Needless to say, I booked it there and then. When I told the rep about the web page price, she said "Oh, the web page is always causing problems and is often incorrect".
Moral here is to resort to older technology and use the dog & bone.
DB