Thank you Megabear2. This is what I generally meant. I knew Saver fare couldn't specify exact cabin, deck etc that's the point of the two fares and I knew it is a risk, and many people aren't willing to gamble on it, but if you can select a general area/cabin grade then we would be happier to book Saver.
I would never expect P&O to negotiate booking a certain cabin after choosing Saver fare, I just wondered if anyone had experience of changing to a different cabin before sailing within the same grading, albeit paying a supplement if it were deemed to be a better general positioning.