The Revenue use a number of test to decide whether you really are self employed, such as whther you do as you are told or whether you do as you wish, whether you take the commercial risk or whether the flying school you work for/with take the risk, among other things. Agree that the IR can collect any PAYE and NIC if it turns out you are employed, but they do have the power, and will use it, to collect it from you instead. They are very hot on spurious self employment (witness the IR35 arguements amoungts cheifly IT people.
If you are self employed you have no employment rights and can be disposed of at the drop of a hat. Also no NMW etc.
Excellent point about insurance. Make sure you have adequate insurance for accidnet negligence etc.
F/o