@stilton
I can't give you much detail I'm afraid - as JT said a few posts ago:
No point asking Clive .. he's an aerodynamicist and, hence, only talks in slugs/cubic foot.
Basically the differences lie in the fine details of the structure. To quote from a Googled article:
Safe-life refers to the philosophy that the component or system is designed to not fail within a certain, defined period. It is assumed that testing and analysis can provide an adequate estimate for the expected lifetime of the component or system. At the end of this expected life, the part is removed from service.
whereas:
Fail-safe designs are designs that incorporate various techniques to mitigate losses due to system or component failures. The design assumption is that failure will eventually occur but when it does the device, system or process will fail in a safe manner.
On the UK parts there were detailed features such as crack-stoppers and multiple load paths whereas the French design relied on analysis and testing to establish where and when any failures might be expected to occur. The consequence was that the ultimate life of the airframe was dictated by the number of thermal fatigue cycles accumulated in the Farnborough major fatigue facility divided by the factor of safety demanded by the airworthiness authorities which was conservative because one was really into unknown territory.