If as you said, is being used on a daily basis aswell as maybe reporting online to DVLA, ring your local police's non-emergency number and inform them that, there is no tax held, yet it's being used on a daily basis and you would reccomend that a unmarked car waits up the road. Just to drop the hint lol. Also maybe add some small white lies to ensure it is acted upon, such as .... the driver never is seen wearing his seatbelt, always tend's to drive with excess speed, tends to often drive in a careless/dangerous manner. Let me know what you do about it, also can you let me know if you find this info/suggestions of any use