Can an employer delay the assessment of a worker with fluctuating earnings until the payroll is run for the purposes of pensions auto-enrolment?

Yes, but only where the employer can still complete its employer duties within the six-week window that it has from the auto-enrolment date for auto-enrolling an eligible jobholder. This may be difficult where the payroll is run very close to the end of the six-week auto-enrolment period. In this situation, the employer may need to predict the worker's earnings instead.