If you employ staff you will need to run a PAYE (payroll) system. PAYE (income tax) is deducted as appropriate from the employee, as is employee NI and employer NI is charged to the employer. This is calculated monthly and should be paid across to HMRC by the 22nd of the following month, however, if the payroll is small may be paid quarterly.
If you have a lot of low paid or part time staff there may not be much PAYE and NI to pay, however, a nil return still needs to be filed. If there are statutory payments to be reclaimed example, SMP (statutory maternity pay), SSP (statutory sick pay) and/or SPP (statutory paternity pay), you may even be in a rebate position, and you can apply to HMRC for this to be paid to you.