As Ethel says, your post is not clear. My understanding is that you are driving a car which you are the registered keeper of and which was bought for you by your ex. You are paying on an insurance policy, which I assume is for the car you are driving.
Does that insurance policy show you as a named driver? If it does then it appears you should be covered. However, was there anything in the insurance proposal you submitted to get the insurance about the ownership of the car? Could it be that you did not declare on the proposal that your ex had bought the car, and the insurance co. has now found out that this is the case and is saying the policy is void?
The above is supposition. Even if it is the case, I can't see why the policy should be void - there must be lots of cases where one person buys a car for another (as a gift, for example which seems to be what has happened here) and the recipient insures it.
If you can't sort it out with the insurer go through their complaints procedure & then to their Ombudsman if necessary.