For the first seven days you are off work you do not need a doctors sick note, you can fill in a self certifying note which you can collect from your employer or download from the www.gov.uk website. If your employer requests a fit note for an illness lasting less than seven days from a doctor the only option is a doctors private sick note for which there is a charge.
After the first seven days of absence from work you will need a fit note signed by a doctor. After the Fit note runs out you are assumed to be fit for work unless you get another fit note. You do not need a 'fit to go back to work' note.
If you have been a hospital inpatient the hospital should have given you a sick note for the time you were in hospital & for any time they think you need to take off after leaving hospital until you should be fit to work.
Fit notes can state that you are not fit for work or may be fit for work with certain modifications for your job for a set period of time.
Fit notes can be issued if you see a gp or if you have been seen by another health professional eg a hospital doctor, practice nurse recently & there are notes on your medical records of their findings which indicate you are unfit for work. A gp can then sign a fit note without seeing you if you phone reception requesting one. Sick notes requested by phone will not be available until the next working day after the day you phone.
For more information see Statutory Sick Pay (SSP).