I have just been trying to change mine, it changed ok but then the alarm kept going off after 10s of locking car, until I changed it back to the original setting (2 for Germany!), which was annoying.
The process I followed is: Bonnet open. Ignition to MAR, then STOP. Push the bonnet switch (little micro switch mounted vertically in the bonnet catch recess on the left) 7 times within 10 seconds (the first push must be made within 15 seconds of the ignition being turned off). While the alarm is beeping, push and hold the switch until one long beep is heard. Release the switch and push again the number of times corresponding to the country code you wish to set it to (5 for UK - no locking BEEEEEP) Wait for a long confirmation beep or a short blast of the selected siren sound to indicate that it is out of program mode. Close bonnet, and lock the doors - silently - woo hoo. BUT then the alarm goes off !!!!!!!
If you work out what I've been doing wrong please let me know.
Also, try and get to grips with the forum search tools, you'll find much more complete descriptions including a list of the different country codes - I know as I did the same search last week. Also, it seems that forum members are more likely to try and help if it looks like you've made a reasonable effort to find the info yourself!
Good luck, hope your alarm doesn't keep going off if you manage to change it
07 3.2 Q4 SV Brera alfa red --- previously '99 156 2.5 V6 for 9.5 years