What low temperature will my small Camping Gaz bottle stop working at?

Camping Gaz cylinders are filled with a mixture of propane and butane. 

This is perfectly ok and your gas appliances will generally work ok with them but because its a blend/mix of both LPG gases, the temperature the cylinders will work down whilst better than just butane is not as low, and as good, as just having a propane gas bottle.  Also its worth noting that the amount of gas plus the small size of the CampingGaz cylinder will have a large effect on the gas flow able to leave their cylinder.

We've never sold Camping Gaz cylinders but from experience of using them personally on small backpack type stoves, we have found they work ok down to around -7 to -10 celcius.  

The bottom line is you will always be better off running propane as this works at much lower temperatures, but do make sure your regulator and gas appliance(s) can run on it before charging to it, or another type of LPG.