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 Gov.uk website
Also click here for commonly asked questions and answers from NHS Choices.
Fit notes and proof of sickness
Employees must give their employer a doctor’s ‘fit note’ (sometimes called a ‘sick note’) if they’re off sick for more than 7 days in a row (including non-working days).
Hospital doctors or GPs can provide a fit note. They may charge a fee if the note is asked for before the 7th day. Fit notes requested from a GP will usually be ready to collect from 3 working days after the request is received. They cannot be post-dated but can be backdated if required.
The fit note will say the employee is either ‘not fit for work’ or ‘may be fit for work’.
If it says the employee ‘may be fit for work’, employers should discuss any changes that might help the employee return to work (eg different hours or tasks).
Employers can take a copy of the fit note. The employee should keep the original.
Another site offering help and support during sickness is FitForWork.org