Some good information in all your replies guys.
Would just like to add the following based on all my research and my invovlement in legal cases.
There is obviously a strong belief/instinct that people have time to jump clear of a rolling over forklift or from a reach truck about to collide with something solid while travelling in reverse. Unfortunately that belief/instinct is not supported by the physics of the situation. I'm not aware of any counterbalance rollover where a driver successfully jumped clear. In one case a young male nearly jumped clear but the overhead guard hit his calves and flung him down onto the concrete so hard that he died of head injuries. And in one other case a very fit footballer nearly jumped clear but the overhead guard got his right ankle and caused severe crushing injury to the degree an amputaion was required.
And I know of five cases with small reach trucks where due to control/ loss of control issues drivers have attempted to jump clear before the reach truck hit a guard/ wall/ racking and ended up with foot of lower leg injuries because they were not able to get to the ground and dodge out of the way in time.
Hence a prudent employer should enforce seat belt wearing rigidly where they are fitted, and ensure where there are no seat bbelts drivers are thrained to brace themselves and hold themseloves within the protection of the forklift structure (arguably there should be sites with mocked up forklift structures which are tilted over and the driver practices holding on)