I found the
CAA IET COP (CAP 1422) interesting informative reading, as also
this FOI request and response. I saw nothing about the power of arrest, or detention. It clearly states that interviews are voluntary and the interviewee should have the opportunity to have a lawyer present.
The relevant sections of the ANO (what a way to waste a morning!) under which the IET operate
clearly state that the the person/organisation being investigated must have "a reasonable time" to provide documents.
It seems to this uneducated oik that they should have had police with them if they felt the need to hold someone against their will (or intimidate them into complying). At the very least, they should have given reasons for exercising their power to prevent the intended flight taking place at the intended time.
I look forward to further education.
While on the subject, is there a list anywhere of aviation-savvy lawyers whom one could call at the drop of a hat to advise at such an interview as our OP reports?