This is another section of the Secretary of State's statutory guidance that I regularly search for:
Paragraph 48"The Secretary of State recommends that approved devices are used only where enforcement is difficult or sensitive and CEO enforcement is not practical. Approved devices should not be used where permits or exemptions (such as resident permits or Blue Badges) not visible to the equipment may apply."http://assets.dft.gov.uk/publications/tma-part-6-cpe-statutory-guidance/betterprkstatutoryguid.pdf