Stopping and starting childcare vouchers

I operate a payroll for a limited company client. The sole employee (director and shareholder) takes a salary of £833.33 each month. On top of this, the employee has been receiving regular childcare vouchers of £200 per month for at least a year. Recently I stopped issuing the vouchers because they were not needed. He is now asking if the vouchers can be issued as and when required for differing amounts.

I am concerned that this will no longer look like a salary sacrifice arrangement if the amounts and timings of the vouchers are inconsistent. What are the rules determining how frequently the vouchers can be stopped, started and changed in value?

Thanks.