Firstly and foremost I am deeply saddend by the death of Henry Surtees.
In reference to you Terance I believe that there is alot more tethers can be improved by.
The material that is used at the moment, in F1 at least, is called polybenzoaoxide and can withstand a load of up to 5000 Kg. However the material breaks easy if twisted or bent...sounds like what happens in an accident if you ask me. The polybenzoaoxide also has to be kept out of sun light.
I can also not remember in recent years any incidents where the wheel has stayed attached to the car in the passed few years apart from Albers at San Marino when he rolled the car over.
Surely this is the upmost thing to look at in terms of safety.
http://www.f1technical.net/articles/64