Ooh, ooh. I know this one.
You have 5 days after it has run out to display it, but you must have bought already.
This is to allow for the post assuming you bought it online.
If you fail to display after the 5 days that it's run out, even if you have bought it and it's on the system as bought, it's still an offence, and you will get done done, probably not by the old bill, but wardens.
But if as you say, it's on your drive, or you park in a private car park, nobody will do you, because if you're picked up on route by the camera cars, it will show up as taxed.
Hope that makes sense, and hope the link helps.
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