From all the years I have tried using p&o discounts and vouchers, they have never worked online and they make your life very difficult when you try and call them as long wait in queues etc. To me, it seems like when it comes to vouchers, they try and word them 'wonderfully' and there's always a reason why they wont work, always in the small writing! So I have resulted in just going for a cheaper company like dfds, even tho dover-dunkerque isnt as good as dover-calais for my route. I usually check price comparison websites when I know what crossing I want to purchase, as I have saved a tenner here and there on some occassions, usually have a look at
www.ferrysavers.co.uk or aferry.co.uk . I remember when MyFerryLink first come out, the prices where so competetive, now it's all about waiting for a good offer via email, so I have found signing up for their newsletters brings you some exclusive good deals too..