Where it's someone driving an insured vehicle but not covered by the policy then the injured parties Insurers will claim from the MIB. The MIB will then attempt to recover the cost from the insurer of the vehicle. As all insurers contribute to the MIB funds from which the claim is paid out I believe there is some sort of agreement that the vehicle insurer will pay out. Think of what happens if a vehicle is stolen and hits another car. The stolen vehicles insurers are usually liable.