Any �mechanically propelled� vehicle when used or kept on a public road must display a valid tax disc. Although there are exemptions to this general rule, none of them apply to your situation.
The penalty for "failure to display" is �60 and is not endorseable.
If you can prove that your tax was purchased (such as a receipt) then you cannot be prosecuted for no tax as you have paid. However, there is a separate offence of 'not displaying a valid disc' and you will incur that penalty.