Ian is a freelance television technician. In a typical tax year he works for approximately 20 separate TV companies. None of his engagements with any of these companies lasts for more than 10 days at a time. All of his work is performed at studios owned by the TV companies, using very expensive equipment also owned by these companies. The TV companies initially book his services by telephoning him at his home (where he has an office) and then send a confirmation letter giving the dates of the engagement and the amount of the agreed fee. Ian's work is very skillful and he is required to do.