What exactly does your Certificate state?
The 'Driving other Cars' extension shown on the Certificate, is, as the name suggests, restricted to cars only - not vans.
So, if the Certificate states something along the lines of "The Policyholder may also drive any other private motor car not belonging to him or hired to him under a Hire Purchase or Leasing Agreement" then you are not insured for this incident.
Should the third party decide to claim, the insurance company which insures the van will be considered the RTA Insurer - that is to say, they are not the contractual insurer (because there has been a breach of contract), but they must satisfy their obligations under the Road Traffic Act. In this eventuality, they have a right to pursue you/your fiancee's father for any payment they make.
Do not involve the police - if you do, it is likely you will be convicted of driving without insurance.
My advice would be to try to come to an agreement with the third party.
If, as has been suggested, this is a 'crash for cash' incident, tell him you are uninsured: I'm pretty sure he will then accept what you have offered him.