When will P21 underpayment be taken

Hi all - I asked this as part of another thread and it didnt get picked up - hoping you can help: I requested 3 years to be p21 balanced- one was correct, one year I was owed money and in the last I owed money. Re the last incident, there were no instructions in the letter received about when I have to pay the underpayment back - does anyone know if it will just be taken from my tax on my next payday...this would not be good!
Thanks

It won't be taken from your next payday. Any changes in your pay arise when Revenue send you and your employer an amended tax credit cert.

How much is involved, a few hundred or a few thousand?

If its not a huge sum involved they probably will offset your refund vs your liability and either amend your 2012 tax credit which will impact on your net pay this year; or incorporate what you owe in your 2013 tax credits.