Ah, it's not that simple.
I believe the Tiger Club have to use 'factory built' Turbulents i.e built by Rollasons, in order that this can be done. They also have a 'letter of agreement' or similar from the CAA which is renewable annually listing the aircraft registrations.
ISTR it was when the law changed (1980s?) and the club successfully forced the CAA's hand to allow them to continue using the Turbulents, the RF4D and the Beta which they had at the time.