P&O ferries rip off!

I am an ex-shareholder of P&O, when they changed hands I chose to remain a "unitholder" - ie retain the travel discount rights in exchange for reduced price for my shares. So in effect they kept some of my money invested in the company, non returnable.
I am now trying to make a Portsmouth Bilbao ferry booking which spans the year end; out 07 return 08. No problem with on line booking at full price..... but when using the unit holder's page in order to get my discount it refuses to let me book a cabin for my return - so rejects my whole booking!
Several emails to customer service, and one to PR dept met with irrelevant replies (no reply at all from PR!)
So what do you think guys, rip off or what!