I was once underneath the Westway in London quite legitimately taking pictures of where the CrossRail tunnel portals would be built and caught the attention of a passing police car. I was questioned and searched under Section 44 which, at the time covered large and unspecidfied areas of the city. The act was later modified as it was unreasonable to expect people to obey the act when it was impossible to know where it applied! Though (as far as I know) it now only applies to 'Government' sites, I would be very careful not to act suspiciously in the vicinity of one and not just avoid entering. Quick tip... If you want to attract less attention when doing a rece...Wear a high-vis jacket and carry a clipboard!