Here's a great way to save even more energy and running costs. Install these PIR light controllers in your corridors and stairwell, and the lights will only come on when movement is detected within a 4.5m range.
The lux or lighting level is adjustable, and the amount of time it switches on is also adjustable, to between 10 seconds and 4 minutes.
These controllers have a maximum load of 1000W of incandescent lighting or 300W of fluorescent lighting or 200W LED, meaning it's easily capable of using it with multiple light fittings.