robin, I guess the point I was trying to make (maybe not sufficiently clearly) was that the reciprocal direction traffic was of a type that would be expected to have a decent (ie mode C) transponder. As you say though (and so does mm_flynn) this may have been an unreasonable assumption, in its own right. That said,I do get the impression that there are a sizeable (minority) of pilots that don't use "C" if it is not mandatory, I think that this is a shame as it is a level of (potential) protection wasted.
As for PCAS and related equipment, I think this is a bit of a controversial topic not least because the earlier units were not so good. However I find my present unit very good (ZAON XRS) – it often detects traffic I never get to see but are subsequently able to verify that the traffic actually does exist from the radio conversations. I certainly would not ignore a warning from it.
(Slight) change of subject: With the availability of lightweight, high power density Li batteries (I use some myself which weigh a few 100's grammes and provide around 100Wh), how valid really is the argument that some a/c cannot use a transponder due to no elec' system? I stand to be corrected but I seem to recall that the average input power into a transponder is less than 20W...