Neil,
As i said, there are two blocks that are attached to the car which the bumper just slides onto. These are on the sides of the car just in front of the wheel arch and are the primary support. DON'T loosen any of the screws on these blocks or you'll rip them them off the car as you pull the bumper off.
If you can, remove the screws around the front of the wheel arch casing that are attached to the edge of the bumper. If not, I suspect the wheel arch will break before the bumper does but keep an eye on these bits as you slide the bumper out. Might be handy to take a tough stanley knife with you to the scrapyard.
Unfortunately, there should be three vertical screws to hold the centre-bottom of the bumper in place. These are evenly spaced (8 inches or so apart) and are only accessible from underneath the car. Mine are missing, but they look like they should be self tappers and only seem to be used to stop the bumper flapping about. With a firm enough tug, you could probably wrench them out of place. As my car proves, it's not essential to have them so if you damage all/some of the tabs, I suspect it wouldn't matter. Alternatively, you could easily come up with another method of re-attaching the bumper here.
There are several interlocking plastic tabs around the front of the bumper that keep it in place. Even if you could get underneath the car, you're unlikely to find them all and would struggle to free them without cracking the plastic. Don't worry too much about these.
The bumper is designed to be slid off forwards, so do as much of the above as possible before pulling. It's easiest to have two people, one on each side, to make sure that it comes off straight and level. It will take some considerable force to remove it, and it won't come quickly. The main resistance will be the innocent-looking screws on the plastic wheel arch casing that I mentioned in para 2.
Good luck! Let us know how you get on.
Richard