I'm surprised at the number of comments blaming the Mail, or making light of the ease of access. Sure, the Mail is reporting in the most hyperbolic way possible - as usual - and given its track record, it's easy and tempting to dismiss the story as simply a tabloid creating FUD. And yes, staff were on site, so you can argue that there was no real threat to infrastructure, if you imagine all staff present are capable of neutralising international spies and/or tooled-up terrorists.
At the very least, it ought to be pretty obvious that leaving the gates unlocked and unguarded seriously endangers the staff. Beyond that, it really shouldn't be that easy for just anyone to get that close to important infrastructure. It's possible to see the Mail's reporting as irresponsible and sensationalist, while still acknowledging that the actual story they're reporting on is concerning.