May I suggest that you post this on the moneysavingexpert (MSE) forum
Insurance & Life Assurance - MoneySavingExpert.com Forums
There are many more experts on that site, who can provide you with valuable advice on this.
My reaction to this is that you should approach the police for a report, which indicates that the 3rd party advised that their car had not been damaged. Then send a copy to his solicitor and Insurer, so they can look into this further. Be careful about making any allegation of fraud without proof.
When you post on the MSE, make sure the post is a bit clearer. I found the events hard to follow. Try to put your case in a date order, so people can see the sequence of information.
Why have Morethan voided the policy, when there was another claim on the policy. You should have advised them of the full information about the use of the car, so they do have a point. Whether voiding the policy was the correct decision, I cannot answer.