Chances are you are here because you have discovered you don’t have a 9mm allen key. Guess what, no one does. As infuriating as this is, the job can be done without the allen key.
This is the same massive brake set-up that is fitted to some of the high-end Audi range such as the S8 and A8.
Here is how to get the calliper off without an 9mm allen key.
Remove the entire brake calliper with the bracket, they are 21mm bolts and take the whole lot off the car. You can now flip the calliper upside and slide the brakes off the calliper and remove the pads, push piston back, clean and lubricate everything and reassemble. Its a little bit fiddly but it can be done.
Hopefully this has got you out of a bit of a bind.