changing immobiliser code

Hi everyone
I have just recently purchased a second hand P reg 106 which unfortunately did'nt come with a handbook. The car has a 4 digit keypad code which at present is on factory settings, i would like to change this for a more personalised number, can anybody help.
Many thanks
Mick.

some of the immobiliser have their code change this way:
1. key out of switch
2. enter code
3. key in switch start car
4. enter new code (with following # or * -whichever your system use)
5. re-enter new code (same here)
6. car off key out.
7. wait untill arming
8. try new code


i hope it did help.