Ill for less than seven calendar days - can your employer ask for a medical certificate when paying Statutory Sick Pay?
No. If you are an employee you can self-certify for the first seven days of your illness. An employer is not allowed to ask for medical evidence (including medical certificates) during the first seven days of absence.
Here is the link from the Government - it uses Swine Flu as an example
Swine flu - self certification and Statutory Sick Pay : Directgov - Swine flu