Stewarding the herds: Pitch invasions are becoming more prevalent, so why don't stewards do more?

It's the 21st of December, 2013, and I've rocked up to Millwall's stadium, The New Den, down in South East London for their Championship match against Middlesbrough. I've been here a number of times before to watch games but this time, I'm pitch-side working at my first shift as a steward.