I'm not a legal mind, but do know we have a "Duty of Disclosure" and the insurers seem to fall back on this responsibility we have. To be honest it does appear that the policy is 'fronted' from outside, not knowing the whole bunch of facts.
Did your father use a NCD against the insurance, or use the fact he had another car to get an introductory discount? These are the sort of factors which switch on alarm bells and get challenged when a claim is made and investigated.
You do need legal advice, for sure. Even a lot of guys in the industry don't seem to fully understand what the impact can be for having a split sort of arrangement, on ownership, registered keeper/owner and who should really take out the insurance. Legal vs. full disclosure are two different things, from my little understanding.