I don't know about getting stoped by the police but can assure you that after 6 months you will now get a notification from the excise people in Tenerife demanding import duty on the car.
The way it works is that the English registration is noted when you land at the port. After 6 months, if you haven't paid the Import Duty the excise people will lookup your details. Presumably this is from the ferry company where you booked with a 50% reduction on your NIE but they might have obtained it by trawling the banks for your name/NIE. Any way if you don't pay they will take the money directly out of your account.

I can assure you that this is the case as it happened to me because of an administration error somewhere along the process. There was a 'Y' on my English plates which was transcribed as a Spanish '7'. Consequently when they looked me up they didn't find that I hade gone through the matriculation process, of which step 1 is pay the import duty. I appealed and was rejected. I was informed that if I didn't pay within a fortnight they would raid my bank account. It took 2 trips to the Import Agency to sort it out and then only because one of the agents kindly took it on to contact them himself.