Thank you everyone for your feedback.
I did try Princess Customer Services again and was told that I have to pay £30 a day extra as my Princess Plus only cost £30 when I booked the cruise last year. I did point it out that Princess should put in their FAQ that "the £20 upgrade to premium" only applies to bookings made where the Plus price was at the new rate of £40 a day.
Customer services did confirm that if I want a drink which is more than the $12 US limit on my Plus package, then I only have to pay the difference + the 18% gratuities. I mentioned that some web sites say that Princess are scrapping this "pay only the difference" but instead you have to pay 100% for the more expensive drink. Their reply was "don`t believe everything you read online " !!
I suppose I will have to see what happens when I get onboard !!
Thank you once again