3 screws and one large hex cap bolt ( 10mm ) each side .
Screws : One in the wheelarch ( this may have been damaged in the incident). One underneath just in front of the wheel. One holding the inner wing plastic cover on.
The two big bolts can be seen by looking down the cutout in front of the headlamp.
You might get away with undoing the bolt on the damaged side only.
What you're going to be doing is feeding the hook in the bumper back over the plastic key on the car.
Have a look at the picture in this post :
http://www.alfaowner.com/Forum/4507496-post18.html (Heater Fan Relay Location - GTV P2)
At the centre bottom, you'll see a zinc plated hex bolt fixing a bit of black plastic to the car's body.
Underneath the bolt head is an angled bit of plastic. That fits into a recess on the bumper. Immediately to the left of the bolt head is a vertically orientated oval shaped bit of plastic. A bit of the bumper slides over that.
If you have to slacken both sides of the bumper, you'll probably need a helper to get it aligned properly on both sides.
Hope that helps.
If you need assistance, I'm near M1 J12. I have a 10mm allen key and I've removed my bumper a number of times!
EDIT : The photo below shows the bracket a little better