Don’t forget to leave the expired tax disc on the vehicle, do not be tempted to put one from another vehicle on in its place.
The offence is for not displaying a valid tax disc, a criminal offence if not displayed, or if you put another cars in its place this is also a criminal offfence. I’ve been taken to court over this and the magistrate threw the offence out as I had an out of date (but valid) tax disc on display.
If you just have an expired tax disc, this is a tax offence and is only a civil matter. You will not be committing a criminal offence (a big difference)!