An outside London ODTC is based on the travelcard price
plus whatever the rail company charges to the boundary
of Zone 6. The ODTC1-6 has gone up by 30p. The Zone 1
tube single doesn't even come in to the equation.

Has this changed? It used to be, the cost of a DR to
London (BR) plus a fixed amount, which just happened
to be slightly less than two single tube rides.
(IIRC 2 quid against 1.10, perhaps someone can
remember how long ago this was?)

Its been a cheap day return plus a one day travelcard for as
long as I can remember.

What you describe sounds like a season ticket to a London
terminal which includes a Zone 1 travelcard. This ticket is
still available.