Bob,
I fitted them without removing the bumper, it's a very easy job, but you need to measure twice and drill once!
Here's an old thread with lots of details that should be useful:
http://www.a2oc.net/forum/showthread.php?5402-PDC-retrofit-an-easy-job
With regard to distances, as Mike has indicated there are markings on the rear of the bumper to show where the sensors should be located, and you should be able to see these through the 'slot' once the bumper trim has been removed. I used these as references and drilled accordingly, for the vertical distance I mounted the sensors in the exact centre of the bumper 'rubbing strip'. If you need help with the distances let me know and I'll measure mine.
There are a couple of routes for the wiring, the linked post uses the grommet at the side, I used a central grommet to get the wires through to the inside then followed the route of the exisiting loom. I initially fitted the buzzer unit beside the control box, but as I have the false boot floor, the sound was too quiet so I moved it above the battery 'hatch' into the spare wheel compartment.
The system works fine and is a very useful addition to an already fine car
Steve