Conjam, this is a popular misconception about SIDs. The complexity of our airspace, particularly in TMA environments, is such that separation will not be guaranteed by relying on SIDs alone. SIDs are designed to get aircraft airborne in a standard way, without the need for individual clearances (and all that this implies). So although separation on a SID is initially assured, intervention after departure is often required to maintain it.
We are always aware of the possibility of radio failure and in these circumstances will co-ordinate with other sectors and units to keep traffic separated.