As you may already be aware, HMRC are rejecting In Year Movements forms submitted electronically by employers who are mandated to join RTI on or before 6 April 2013 with error code 7818 stating "This PAYE In Year Movement submission cannot be accepted as the employer has been invited to join RTI"
Guidance for Employers who need to report details of employees leaving:
Instead of submitting P45's the employer should enter the leaving date on the employees’ 2012-13 P14.
If the employer has already submitted their 2012-13 P35 and P14's without a leaving date then they need take no further action.
The employer should not include the employee on their first Full Payment Submission (FPS) or Employer Alignment Summary (EAS).
The Employment will be then be automatically ceased at 05/04/2013 following HMRC’s processing of the employers first FPS or EAS.
They should not submit revised P14’s.
Guidance for Employers who need to report details of new starters from the previous tax year:
Employers should include details of the employee in their EAS and/or first FPS and either show a date of starting of 6 April or leave this field blank.