The rear one is quite easy, the front one isn't too bad either, just a few more bits to to take off, to gain access, but held in a similar way.
The rear one is held on by both nuts screwed on to the bolts moulded into the bumper which come through the back panel. There are also two (one either side) securing bolts on the side which allow the moulded channels on the bumper to slide on and when tightened keep the ends of the bumper flush with the body.
Bonzo
December 2004