If you are absent from work less than 7 days, including weekends, a Self-Certification Form can be obtained from your employer or by visiting www.hmrc.gov.uk/forms/sc2.pdf. Med 3 certificates are only required for absences of more than 7 days in a row and will not be issued for shorter periods.
Full details regarding this Government directive can be found at www.gov.uk/taking-sick-leave.
If you require a sick note for a shorter period of time it will be subject to a charge – see non-NHS services.