My car ran out of tax today so i need to re-tax it.

If i buy my tax online can i drive around without the updated tax disc until the new tax disc arrives?

No you cant i'm afraid. It is an offence to drive your car or even have it parked on the road without an up to date tax disc. You can get fined for not having one. Not really worth the risk!

Ah right, its no bother, i'll just go to the post office and get it there and then tommorow.

No you cant i'm afraid. It is an offence to drive your car or even have it parked on the road without an up to date tax disc. You can get fined for not having one. Not really worth the risk!

Not just a fine, the car will be seized and crushed.

a m8 of mine taxed his car over the phone .... drove around that evening with the old tax disc in..... copper pulled him and the outcome was a 60 quid fine even though he has technically got tax

The offense is failing to display a valid tax disc for that car. I dont yet see any point in buying one online.