When providing avoiding action to a military fast jet it is often better to go for height separation rather than a turn as anyone who has seen a GR4 trying to turn at high speed will testify to. Getting at least 3000ft is sometimes far quicker than getting 5nm. Equally, providing avoiding action within a known traffic environment indicates that something has gone wrong with the system already. Outside of CAS avoiding action is aimed at, where possible, achieving 3000ft or 5nm from unknown ac. These ac may or may not have TCAS, which may or may not agree a safe course of action based on predicted profiles of both ac. Additionally, the conflicting ac may or may not have SSR at all. Furthermore, TCAS and radar separation are based on two entirely different principles, an ATCO may give avoiding action at 10nm knowing the turn or climb will achieve 3000ft or 5nm, TCAS however will only kick in based on time to collision and profile, this may or may not be at the same point the controller issues the instruction. I personally see no conflict between the two systems when TCAS it is used as a last ditch effort to prevent a collision. However, TCAS does fall down when RA's are generated between ac that are climbing and descending to cleared levels, which provide separation, but due to the ac's conflicting profiles and the systems inability to grasp this simple ATC concept, false or spurious RA's are generated. These RA’s may place the ac in question into far greater danger; the machine may be good but as with everything, people are involved as well.