Got mine today as well.
The Cardboard with P on is for car parking (similar to last year) but go to South Bank anyway if you want to, rather than where directed. May be able to blag on to there for free on Saturday if open, but you might have to pay a fiver on the Sunday.
Although the letter only refers to the Saturday hot laps experience, the Plastic pass on the necklace must be a Seat Guest paddock pass getting you admission to the circuit both days, and into the paddock for the Saturday at least. As others have said bring the pack and letter and remember to be nice to the jobsworth's and you should get to anywhere you need to be.