Well you can fit it without removing the seat.
You get 2 brackets and 4 Torx screws.....
The first pain is the 2 brackets are not tapped, you will need to run the screws through a couple of times to make the threads. Hold the bracket in a set of grips....
The next stage is to raise the seat up as much as you can....
If you look under the seat you will see a strap that runs along the length of the seat, it has 3 holes in it.....
Again use the screws on the 2 extreme holes to make a thread..
Next fit the bracket to the seat, the tab on the back holds it straight.. This will take you about 16hrs, give you brain ache and make you want to give up.
Once you've got the brackets fitted, it's easy to fit the extinguisher bracket and strap. Note position
Then snap the extinguisher into the bracket, make sure the 'handle' end is facing the door.
Thats it........