Employer returns: regulation 80 determinations: time limits and appeals: regulation 80(5): parts IV and V TMA 1970
The rules about time limits and appeals apply to a determination in the same way as an assessment, so you should apply them as if you had assessed the employer.
You may sometimes need to determine tax for a year outside the ordinary time limit.
Section 36 TMA1970 provides the authority for determinations outside the ordinary time limits at Section 34 TMA1970, where there has been a loss of tax attributable to careless and deliberate conduct.
COG914175 contains advice about determinations made outside of the Ordinary Time Limits.