Thank you very much for that. Turns out that the off-peak cap doesn’t mitigate the ‘Evening Peak Trap’ for people who travel towards London offpeak. Looking at the live Chiltern rates, the offpeak caps are not generous and are typically the same as or more than the total of peak+offpeak single fares if no tube journey is involved. The off peak cap seems to be set at the offpeak travelcard rate on Chiltern.
However, there is another way of avoiding the Evening Peak Trap (aside from the Offpeak-with-travelcard option). This is to buy an old fashioned day return ticket from further back down the line!
For example: Sevenoaks-London Terminals, travelling out after 9:30ish and back 16:00-19:00:
Offpeak plus peak singles: £8 + £14.60 = £22.60 (cash or contactless)
Offpeak with Travelcard: £20.80 (cash)
Offpeak day return from Hildenborough £18.70 !
Thus sadly, the evening peak fares further reduce the usefulness of the contactless system.
Btw on our line, many passengers can travel to London without the need for the Tube, thanks to the location of Charing Cross and the many main line stations that we can travel to. So there is less need for the Travelcard benefits.