How long have you had the car? Anyone you live with had any arguments or fallout?
Some guy on Detailing World had this happen to him. He bought a mini cooper s I think and within two weeks of buying it (brand new) someone poured the same stuff over his car. He went through insurance and had issues with the "approved bodyshop". After going through one of his own bodyshops, he got his car back. Three months later! Anyway, turned out his daughter, who hung out with some questionable characters, had been accused of sleeping with someone else's ex-boyfriend. This bunny boiler thought the mini belonged to the daughter, did it to get revenge, and then openly revealed to her own friends what she had done. Police were no help.
I'm gutted for you mate. I hope you get the scumbag, soak them in nitromors and then set them alight. Nitromors is industrial strength so someone would have gone out of their way to do this. It was premeditated.