Without the code the unit is no good . the codes are on the Car Pass card . (Which i understand you don't have ) You can buy a new car pass from a Vauxhall main dealer , they cost between £25 - £50 I think.
You need proof of ownership & ID to get one of these because it is unique to your car .
you will also need the CDR 2005 manual ( probably downloadable from the net)to show you how to submit the code to the unit . it involves switching on ignition , switching off unit & pressing certain keys as you put in the code .
If sequence is not followed it will not work & you only get 10 chances to "code" it . Everytime you put in an incorrect code ,the time you have to wait increases . . . by the time you reach attempt 7 the time wait is hours.
Someone on here who has more knowledege of this unit will advise you in more detail . . . but for the time being don;t mess with guessing codes

DaveDND is the man

Try a search on the net for DND Services Ltd They are in the Torquay area somewhere
Always worth contacting previous owner . . . he/she may have the Car Pass
Update: looked on the DND website . . they can decode it for you for £15 .00 but you have to send it to them . 48 hr turnaround
website also shows you sequence for loading the code