How to fix a car radio after my battery went flat?

The battery on my car went flat and my radio no-longer works. The code to unlock the radio is not in any of the car documents and because of the cars age the dealer does not have the information on file.

I have a Vauxhall Corsa Trip, W reg and the radio is typeR100-CAR200. I have seen several internet companies offering caodes for cash but most of them need you to send the radio in which I'm not keen on.

Help would be appreciated.