Less than 7 days
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.
More than 7 days
A ‘Statement of Fitness for Work’ (fit note) from your doctor is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise. Your employer will decide whether or not this evidence is acceptable. If your employer has any doubts, they may still ask for more information from your GP. 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.
An SC2 self-certification form (SC1 for unemployed or self-employed) is used by an employee to provide their employer with details of sick absences of 4 or more days in a row. The completed SC2 form is then used to help decide whether or not the employee is entitled to receive Statutory Sick Pay (SSP). GPs are not obliged to issue NHS medical certificates (Fit Notes) for periods of sickness of less than seven days’ duration. However, if a patient requires a short-term certificate, the GP is entitled to charge a fee. However, it is clearly not an appropriate use of NHS time to see patients for no other purpose than to provide a sick certificate.
If you are sick for more than seven days, your employer can ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay). It is up to your employer to decide whether you are incapable of work.
click on this link to find out more http://www.direct.gov.uk/en/Employment/Employees/Sicknessabsence/DG_187161