They are clipped in. If you can get your hand behind to release the clips you don't need to remove the bumper. If you have a bodykit sometimes it has to be partially disassembled to get your hand to the sensor.

If it is the same sensor as E46 and E39 you can get a brand new land rover sensor for the Range Rover L322 1999-2004 for 25GBP on eBay.