Coldest I've ever seen was -5c, -10c is pure evil.
At what point does petrol freeze?
Petrol begins to freeze at -71C, although if there is water in your tank, the water can freeze and block fuel lines/pumps etc... But the petrol itself... very cold.
Diesel on the other hand starts to cloud at 0 degrees, and begins to gel at around -10 degrees. However the diesel you buy in winter is often a different blend with better low temperature performance. So unless you drive really infrequently it shouldnt be a problem.
Some diesel vehicles preheat the fuel to prevent gelling.