I've just had a look in the Haynes manual which isn't terribly helpful on the subject.

I think there's only the bumper and it's attached shock absorption pad. If you take the bumper off then I don't believe there's anything else there. There's just a big gap in the bodywork where the bumper fits in.

Sorry, that's not very helpful, is it!

Some have polystyrene packing between the bumper and body to reduce noise,but it usually gets left off.Otherwise it bolts right onto the irons.

The primary purpose of these blocks is not to reduce noise but to absorb energy in the event of an impact. The blocks are usually EPP (expanded polypropylene), not polystyrene. You'll find similar foam blocks inside doors and behind various trims, again to absorb energy in the event of a side impact or head impact internally.

If the car is supposed to have an EPP block behind the bumper, don't leave it out. Doing so reduces the crashworthiness of the vehicle and may result in preventable injury to occupants in a crash. They may also provide pedestrian protection (a big issue in crashworthiness at the moment - it's one of the drivers for side pivoting wipers to improve pedestrian head impact protection across the bottom of the screen area).