The worst thing (if you get charged) is that the 5 GB does not "carry over" to the next month.
However you can set the limit, for any "extra" usage you are willing to pay for, down to 0 pounds.
Then, if you exceed the 10 GB allowed on the "Value" account, PN limits the speed of downloads to 128 kbps until your next billing date. So e-mail and general browsing is OK, but downloading a TV show on iPlayer would take a long time and YouTube videos would be forever pausing.
However, 10 GB is reasonably generous and it should be fine for general browsing and e-mail.
I (and a few others) have been pushing for there to be a more generous allowance for the Value account. On "Premium" they allowed 80 GB, but that a/c was replaced by "Extra" in April, and at the time the "Value" a/c was restricted to "up to 8 Mbps" and 10 GB.
Now it is "up to 20 Mbps" but still 10 GB, and I feel strongly that the jump from 10 to 60 is too big a step for a household wanting 15 to 20 GB. They might choose TalkTalk or O2/Be if available, as those ISPs don't have a strict limit on most accounts.
It only costs an extra fiver a month to get 60 GB, and don't forget that if you want to download shows with iPlayer you can do that from midnight to 0800 without it counting against the 10 GB/ 60 GB allowance. Last month my traffic hit well over 150 GB (90+ GB overnight) but I am a heavy user (also use 15 GB on 'Three' mobile).
(In case anyone wonders, I have set up some linux PCs to act as backup storage for clients, so they can copy important files into one of mine, on a daily basis). I am also often online at night, downloading iPlayer shows sometimes, and streaming video other times.
I am even tempted to change to one of the EDF electricity accounts which is cheaper from 9pm to 6am and then sleep all day :)