You may already be aware of the above, and implemented safety measures to protect against any incidents, however, if you are not sure then please spare a minute to look through this site which is certain to offer a cost effective solution to many fragile roof situations.
The Health & Safety Executives Construction Division led a campaign in 2004 relating to falls from height and now expect to find...
1) - An assessment of risk where someone could fall from a height
2) - Roof lights and other fragile materials protected to prevent falls through the material