As CF said, it takes about 8 to 10 hours before you get another 4 chances to input the code. The igniton NEEDS to be on for those 8 to 10 hours. so you will have to wait until you have clocked up that ammount of driving time. The radio must be TURNED ON for the time to elapse.
The standard fit to an L-reg Corsa would, by today's standards, be pretty s**t judging by what my J-reg Astra had. Just go to Halfords and get a half-decent unit for a handful of coppers.