Rod, it's indeed possible that your picture is closer to the mark for weekend traffic. I rarely fly at weekends.
My comment about the correlation between Mode C carriage and altitude is based on an obvious observation of traffic reports by radar ATC.
I tend to fly either just below CAS or (if CAS base is in IMC; often the case under the LTMA) just below the cloudbase, so there will be negligible GA traffic above me. The vast majority of traffic reported to me in this situation is "level unknown". Most of this traffic is never spotted (by me or passengers) but when it is spotted it turns out to be way down below.
If OTOH I fly higher up, 3000ft+ which a lot of the time is above cloud, the vast majority of traffic spotted is reported with a known level (Mode C).
Anyway I have written this before many times. Maybe everything changes when the weekend comes and suddenly all the non-transponding pilots climb to great heights?
Transponder use is (as you well know) not just for the benefit of the few TCAS-equipped Cirrus SR22s etc. You get
- protection to jet transport TCAS, should you bust CAS (of course you never bust CAS but hundreds of other pilots manage it every year, with plenty having near misses with 737s etc)
- protection to jet transport TCAS in Class G (the only solution to CAT in G is more CAS, which nobody in GA wants, do they??)
- radar ATC can see your level and pass it to other pilots (hardly any point in calling any ATC unit if they cannot provide a radar service, but a RS is near-useless without it seeing altitudes)
- easier to get CAS transits (apart from airspace rules, a transponder makes you appear more professional)
- visibility to other planes that carry TCAS (fixed or portable) systems
Considering the average 30 year old spamcan costs about £5000/year to drag through the Annual, I don't really see this argument anymore. And the vast majority of "homebuilt" planes can have one fitted too - even if battery powered.