car insurance problem

a friend of mine put his father on his car insurance as a named driver a few years ago as it helped reduce the premium - the father never drove the car as he has his own. A year later the father got an SP30, and my friend informed the insurance company by letter. He never heard back until now, 4 years later when the insurance company have somehow noted the SP30 and sent a bill for £1200 for extra premium for the preceding years. My friend contacted the insurance company who say they never received the letter, though he does have a copy of it.

so.. does he have to pay the £1200? What if he doesn't? The insurance company say the car was on cover for the name driver, but surely, without additional premium being paid for the SP30 the insurance was invalid for that driver at least, and i'll bet in the event of a claim they wouldn't have paid up..

any comments as i reckon a few people with named drivers on their policy could fall into this trap