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 by clicking on the link below and printing off a copy.
Self Certification
For any illness lasting longer than seven days you will need to see the doctor for him/her to issue a sickness certificate, now called a "Fit Note", and for any subsequent renewal of the certificate.
If you have had an operation or procedure done in a hospital by a consultant, requiring you take time off work, you should be able to get a "Fit Note" directly from them. Enquire before you are discharged or at your out patient's appointment.
For more information on the new "Fit Note" please click on the link below from the DWP.
http://www.dwp.gov.uk/fitnote/