One of the posts on the older thread on this topic has reminded me of something.
I'm fairly sure for one year in 1985 (perhaps 1984 also) there was, in addition to the Caterham/Tattenham services, a Charing Cross-Reigate or Redhill service hourly during the day.
I think it went the Sydenham route and it provided half the Coulsdon South/Merstham services, the other service being provided by an (I think) London Bridge-Reigate or Redhill via Crystal Palace service. Consequently it wasn't perfect clockface half-hourly. Never saw these "in the flesh" but remember seeing them in the 1985 timetable, which was probably the most cut-back timetable of the entire 80s.
Can anyone with access to the 1985 or even the 1984 timetable (could well have come in with the GatEx timetable, not sure) confirm this?
It was definitely gone by 1986, when the Horsham services started calling at Coulsdon South and Merstham, having skipped them earlier in the 80s.