Does anyone know what the law says about being off sick?

Is it 3 days of nothing before your entitled?
If your in Full Employment.

In order for you to claim SSP (Statutory Sick Pay), you must be sick for at least 4 or more days in a row (including weekends and bank holidays).

If you earn £84 a week or more before tax and national insurance, you are entitled to £70.05 per week.

You will get this paid to you instead of your normal wages and you will not lose any holidays.

If however, you are sick for less than 4 or more days in a row, you are not entitled to SSP. Thus, it is an agreement between you and your employer as to if you should lose your wages for the day or days you have been off sick or instead class your sick days as holidays.

More information can be found here