heres from the tax website.the cossie is ok!
If you use the electronic vehicle licensing service or tax by post at the end of the month, there’s now an exemption for not displaying a tax disc. This exemption covers the first five working days of the month to allow time for the new disc to arrive in the post. While you are still waiting for your tax disc you will need to display your current tax disc. The exemption only applies if applications are made before the current tax disc or SORN expires.