March 2012 P45 received in April 2012. How to deal with this?

Employee started job on 21/3/12. No P45 given, so BR tax deducted on 31/3/12.

P45 now received in April. Had it been received on time in March, no tax would have been deducted.

How can the tax paid in March be recovered in April, seeing that it's a new Tax Year? Or should the employee complete a SA Return?

Thanks