Last time I had to go off it was a sick note, it was only a year or so ago.
And it was basically just a bit of paper filled out by the doctor "Pitchfork has a shattered hand etc."
It was a fit note, though in all essence nothing changed from them being the old sick note, just the word 'fit' on it

Department of Work & Pensions said:The fit note was introduced to replace the old sick note on 6 April 2010. Doctors issue fit notes to individuals to provide evidence of the advice the doctor has given about the individual’s fitness for work.
The fit note allows doctors to advise that individuals “may be fit for work” taking into account the doctor’s advice, or that they are “not fit for work”.