You can do either.
You can call up the PAYE helpline (1890 777 425 - this is the number for my area - it may differ where you are) and she can request her credits to be transferred to you.
Or you can leave it until 2012 and request a P21 Balancing Statement.
If you do decide to transfer over her credits now, it would still be worth ordering a balancing statement in 2012 for 2011, as you cannot transfer her PAYE credit across and so there may still be overpaid tax. It is also worth mentioning that Maternity Benefit is tax free so there may be some overpayment from her maternity leave.