Tax Code applied too late?

cobbles

I process the payroll for a client once a month at their premises, they leave a file out with all the notifcations from HMRC or Sage info the lady that normally looks after the file was off sick for a couple of months, the company received new tax codes for 5 employees in December 2010 but they were not given to me until this month - the notices note that the tax codes should be used on the next pay day which should have been Jan 11 not Mar 11, should just apply from Mar 11 or should I re-run the two previous months?

Monsoon

If the tax codes are cumulative they should pick up the change in tax.
If the tax codes are not cumulative, not sure, might have to re-run.