Outsurance declining a Third Party claim

I have had a third-party fire damage claim against an Ousurance Client rejected today because the insurer believes there isn't enough evidence that their client was negligent or his negligence caused the fire that burnt parts of my vehicle. Please note that their client has admitted to his car causing damage to my vehicle and has been honest throughout the process of getting this sorted. Outsurance says that as long as the law exists that says negligence has to be behind the cause of the fire for them to entertain third-party claims of this nature they won't bulge. Long story short, my car was parked under a covered parking area in the office where we both (me & the Outsurance client) work on the 28th of April 2023, to my knowledge only my car was parked in this designated area on the day because most of this employers staff were given a day off for that Friday leading to the Long weekend & only their Rentals division came to work on the day because of the nature of the business they deal in (Mining & Construction) meaning that I had to be at work being their IT services provider, to my surprise it came to my attention later that another car has parked next to mines when I suddenly heard a huge explosion (sound) coming from this parking area just next to the office block where my IT office is. When I ran outside to do an investigation I found the Outrance clients' car fully engulfed by a huge flame spread towards the parking bay where my car was and raging towards the office block where I was. 3 more explosions would follow burning the Outsurance client's car to ashes as I and the security team present on the day tried to contain the fire that would give me a chance to pull my car off this parking and also prevent it from burning further and the fire from reaching the office building. I quickly took the risk when the fourth explosion subsided & jumped into my car which was already burning on the entire left-hand side, by luck the car started and I drove it back to safety. Today after lodging a third-party claim, Outsurance is telling me that it has been declined because of insufficient evidence to suggest that there was negligence on their client's part and that their specialist only found the battery on this car to be the cause of this explosion, this is a car that was literally burnt to ashes. Now my next move is to take this to small claims court as I believe the handling of this whole claim on the third party side has been unfair, and so you know Outsurance third party does not cover the cost of damages as they say because of a common law that protects them in such cases, why any person would take out third party cover with them after this is beyond me. But anyway our experiences are not the same. they have not sent me the report that suggests that the battery caused the issue, I am interested in knowing how this conclusion was drawn from the ashes & the state this car is in & also why the protection of their interest comes before that of their paying clients & victims.