Thanks for the responses : the consensus seems to be that 7700 to alert lower airspace is a good idea . ( Although more opinion is always welcome )
So here is a supplementary .
I have also suggested that in a radar environment - everyone on Headings or Direct To's rather than on airway centrelines - an automatic turn away from present heading might do more harm than good : who knows where anyone else is . On the other hand an immediate "MAYDAY MAYDAY MAYDAY <callsign> Emergecy Descent , Request Heading" might be a useful aid to avoid smacking into someone else .
( Also , TCAS to TA only - and BELOW - might assist , although pilots should all be aware of the dangers of basing Collision Avoidance Heading changes on TCAS returns , as explained in a recent Eurocontrol ACAS Bulletin :
http://www.eurocontrol.int/msa/galle...n_6_Mar-05.pdf
Are these useful too , or do they make it worse