The NHS provides sickness certification only after seven continuous days of illness.  If you need a sick note to cover this period of sickness you should complete a Self-Certification (SC2) form which is available from your employer or on the HMRC website. 

Please visit the Direct Gov website for full information about fitness for work following a period of sickness. After this period a NHS certificate may be issued but you will need to contact the surgery as it may be necessary to discuss your request with your GP.