Your boss may try to persuade you to go to work if you do not have a sick note and you haven’t been sick for more than 7 days. However, you are not obliged to go to work if you are feeling sick. If you have been off for 7+ days then you need a sick note from your GP so you can remain off work. If you have a sick note for 5 days but you feel better on day 3, it is really up to you if you want to go back to work or not.