A fit note is the informal name for the Statement of Fitness to Work. Fit notes replaced sick notes from 6 April 2010.
The rules on fit/sick notes:
If your employer wants a doctor’s note for a period of illness less than a week we are not obliged to issue any certificate. Many employers have their own self-certification forms. If your employer doesn’t have their own form, instead they may use an SC2 form from HM Revenue & Customs: Employee’s Statement of Sickness (PDF). In special circumstances we can issue a private sick note but this will incur a fee.
For illnesses and injuries which have involved the hospital or A&E (such as a broken bone) we are able to use consultant letters as proof of illness and can in many cases safely give you a sick note without you having to come into the surgery. The hospital wards and A&E can issue you with a fit/sick note if you ask them at the time.
If you have a long term illness we are often able to do a fit/sick note from your records without you having to come into the surgery to be seen.