A small fire extinguisher hitting you on the arm as it then travels through the dash and windscreen, probably stopping halfway through whatever you just crashed into, will be the least of your worries.
I fitted mine in the Dax, between the seats on the rear bulkhead (careful you don't drill through the battery...ala Rodders).
Not sure where to fit it on the Crendon though. Probably in front of the seat like Lee says.