Sorry Sarah, youre wasting your time with the police. Use GOV.UK to report. Then, satisfyingly video the truck that removes the untaxed car some weeks later.
The only option is probably to get a lockable post fitted. The only other thing is that the number plate will now be searchable on Google. Anyone thinking of buying this car will probably want the reg changed = hassle and expense. If you can post it on Facebook and Youtube, even better.
Not that good of a driving instructor.