You are in luck,I have my daughter's '97 corsa manual to hand.I will copy the method as it is shown.
1.switch on ignition.
2.switch off the radio.(big round knob on left)
3.press and hold"AS"button while switching on radio,display shows "CODE",but after 3 seconds should change to ----.
4.enter the code # using the station preset buttons.
Example--your code is 3650,press button 1 3times,press b2 6 times,press b3 5 times,press b4 10 times to get 0.
5.store the code by pressing "AS" button until the set bleeps.
You should now have some music!!
Take care not to enter code incorrectly as each go extends the waiting time,before you can try again.Good luck.
For information,the philips sets in vauxhalls are recoded the same way,except you push the buttons once for 0 and twice for 1 etc.