The Inland Revenue are of the opinion that if as an FI you only work for one company and are required to turn up for work 9-5 then you cant be self employed.
Some companies feel that they employ FI's only when they are flying not when they are supervising solos, briefing or waiting for next student or A/C. The inland revenue say that if you are required to attend a place of work you are employed for the time you are there, and should recieve the minimum wage for that time.
Good luck MP speak to you soon.