Policy taken out on car I own by deceased father.

My father originally took out a netstar policy on a car registered in my name about 8 years ago. The car was bought cash. He passed away about 2 months ago and I made the decision to sell the car as he bought me a new one, cash, right before he passed away. Some of his netstar policies have been altered in the past by my stepmother or his company with no problem. I phoned and was told I need to supply several documents, including letters of executorship despite the fact that the car is in MY NAME, was bought CASH and about to be sold. My stepmother phoned several times to speak to the contact person and e-mailed the contact person and nobody is helping us clear up the matter. The policy's fees are deducted from the company which is run by my stepmother and in which both of us has shares. Why do we not have the authority to cancel the policy? I HAVE SOLD THE CAR and would like assistance in CANCELLING THE POLICY if someone can PLEASE HELP ME WITH THAT. I include my deceased father's ID number in the details below as the policy is in his name.