2) WHY DIDNT THEY BACK IT UP AND FORGET A DISK?
That's not the point. I think it probably is backed up. The problem is where the data has ended up - probably with some criminal organisation with intent to commit fraud. This data included full names, dates of birth, addresses, and worse, bank account details and national insurance details. All very easy for someone to potentially commit identity fraud.
The brainless numpty who dispatched said envelope obviously didn't realise that not all Royal Mail workers may well not be honest as they should be. I'd certainly be intrigued if I received a batch of mail from a goverment department including a plain (ie, not recorded or special delivery) envelope obviously containing disks if I was on not much more than the minimum wage in a sorting office.