You do not require a doctor's fit note for any illness lasting 7 days or less.
If you're sick and off work for more than seven days, your employer will probably ask for a fit note as proof of your illness. Fit notes are also known as a ‘Statement of fitness for work’ or a ‘Med3 form’. Doctors issue fit notes to record the advice they have given about their fitness for work. With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury. They are also used to support payment of SSP (statutory sick pay).
For more information see the DirectGov website (where this information was sourced)