What is the seven day rule?

This is a special exemption, just for film and TV, which allows people who would normally be PAYE to be paid gross. The company still has to pay the National Insurance though.

2.3 THE “SEVEN DAY” RULE 
Because many employed workers in the Film, Production & TV Broadcasting industry have short engagements with a succession of different employers, the normal operation of PAYE is impractical and would in many cases result in excessive deductions of tax. The Seven-Day Rule is intended to alleviate the hardship, which might arise from such excessive deductions.

You need not apply PAYE to payments made to workers engaged for less than one week – that is, for six consecutive days or less.

The period of six consecutive days includes rest days and weekends if these fall between the first and last days of engagement. For example, if a worker is engaged for
Wednesday, Thursday and Friday and for the following Monday and Tuesday, the intervening weekend must be counted and the limit of six days will be exceeded.

Here’s the most recent copy of the guidance document issued by HMRC fi-notes-2012