Hi all,
just a couple of details here. You will have to display a valid tax disc else you will commit an offence. The disc in the pocket is not exempt. And you have 5 days grace to allow for the new disk in the post but you must still have the old disk displayed.
If any police person can be bothered to fine you is another question. I was stopped when I lost one without realizing and they were happy to just check and make me aware of the rules.

https://www.askthe.police.uk/content/Q665.htm

Rob