So what we've basically concluded is that PCAS is useful, but has limitations (lack of transponders in many aircraft, unreliability), but FLARM seems to work (in a limited range), the only obstacle being that the idea may never catch on to the extent that there is a point in having one installed.
So a rather contravertial idea - how about petitioning the CAA to make FLARM (or a similar system) mandatory? Midairs would become a thing of the past, and it would greatly improve the image of GA in many eyes. At 500 pounds a shot (or much less when demand picks up) this would seem to be a very real solution to this massive problem.
Who's with me?