micra - radio unlock?

Hi,

I recently bought a Nissan Micra (P Reg), with an RDS radio system. Over the weekend, the battery died, and when restarted, the radio wouldn't work, and I don't have the code to unlock it! The previous owner also didn't have.

Qn: How do I find the serial number of the radio? (Have looked at it, and there are no numbers on sight)Do I nee do to take the radio out?

Qn: From there, how do I go about getting the unlocking code?

Can someone please help?!?! Many thanks!!