Thanks for the advice guys. I think I'll take Haraka's advice - seems to be the easiest solution.
You are right St John's Wort. I was once sitting in my old Zephyr convertible with the hood down near the airfield at Seletar taking photographs of whatever was airborne at the time. A Land Rover pulled up and the WO i/c RAF police gave me a polite but firm bollocking for taking photos on MOD property without a permit.
Luckily he didnt insist I hand over the film, so I was able to keep my photos of highly classified Beverleys and Twin Pioneers!