You do not require a doctor's sickness certificate for any illness lasting seven days or less. Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website
For any illness lasting longer than seven days you will need a fit note (Med3) issuing by a Doctor.
Please call reception if you need a fit note for work - they will assess if you need to speak to a Doctor for the certificate to be issued. Please follow the same procedure for extending a note
If your employer insists on a sickness certificate for 7 days or less a charge will be levied
If you have been told to self isolate because of coronavirus and you need a note for employer, you can produce a self certificate via this link: NHS isolation note