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 empolyer or click the link below.
Self-certification Form (SC2)
If you're off work sick for more than seven days, your employer will usually ask you to provide proof that you've been ill. They will normally ask for a fit note from your GP. These are signed in the surgery every morning Monday-Friday at 11.15am.