If you are off work for seven days or less, no certification is required. However, you may need to complete a self-certification form, which is obtainable from your employer.
https://public-online.hmrc.gov.uk/lc/content/xfaforms/profiles/forms.html?contentRoot=repository:///Applications/PersonalTax_iForms/1.0/SC2&template=SC2.xdp
If you are absent from work for seven days or more, because of illness, you may require a doctors' certificate. In which case, you must arrange an appointment with a doctor, otherwise a certificate will not be issued.