I do cranes and stuff almost exclusively on my own. I can't speak for Manchester but where I'm from, despite there being a decently high level of security at all sites, actual security themselves are slow to notice and slow to react. In probably 15 attempts I've had a security response in 2 of them. First one was just a loudspeaker saying they're calling the police as I was climbing back out over the fence and the other was just a couple of guys walking about that clearly didn't want to actually encounter me so it was an easy walk to the exit. Your mileage may vary though, obviously.
I remember being nervous waiting for a good opportunity to hop the fence on my first solo mission, but at a certain point you've just got to either go for it or go home. If you do decide to go for it, good luck and be careful!