Be careful.. The 14 days grace period that you hear about is nonsense. There was a 5 day period to cover postal of the tax disc, but that was 5 days from application to delivery, now they're doing everything online with Tax, Mot and Insurance you're more likely to get f**ked sooner than you expect it.
Technically, and legally you are in liable from 00:01 on the day after your tax expires. The delay in getting your notification is down to human administration. Not so much now its automated. Your car will be flagged up and the £80 issued automatically.
With the ease of applying for Tax and SORN online, you'll have no excuse.