Hippy,
Below is the email for my contact at the SIA. He is the one writing the requirements for PI. If you email him your exact circumstances and examples of the sort of work you do, and details such as are you employed (rather than hired) by the company and are the people you are investigating also employees. Then he will get an answer from the legal person who is working on the legislation.
Needless to say this address has to be used sparingly, so don't forward it to people or post it on the forum
Tony.Holyland@the-sia.org.uk
Nina