25.This section provides that all conduct defined in section 1 will be lawful, provided it is carried out in accordance with the authorisation to which it relates.
26.Furthermore, no civil liability will arise out of conduct which is incidental to the authorised conduct. However, this does not apply in the case where the incidental conduct is conduct which itself should have been separately authorised either under this Act or under Part III of the Police Act 1997 (which relates to authorisation of interference with property and wireless telegraphy).
Commentary on Sections Authorisation of surveillance and human intelligence sources Section 5:  Lawful surveillance etc.