After years of trying P and O on the phone and actually breaking one phone in frustration at being kept on hold, I have switched to emailing; the customer services form or address is on the My P and O cruises website.
I'm all sorted for my 2 back to backs on Britannia starting on Friday, but wanted to look at excursions for the next cruise on Aurora. I couldn't get in; the personaliser just kept saying "invalid booking reference" so I emailed.
I got an answer the same day suggesting that the problem might be my name being misspelt on their records for that cruise, and asking me to confirm all the names on my passport in correct order. They then came back the next day saying they had corrected their records and I should be able to access my personaliser. Great service, and it worked!