I've complained before that NSE's integrated rail and tube fares fall
apart once one stays overnight in London (as I am tonight) but if you
have to travel to London in the morning peak it gets more strange.

Normally when I stay in London I get a Saver Return from Cambridge,
either to London Terminals (£16.55 with Network Card discount) or
Underground Zone 2 (£19.15). This is because a Saver Return to Putney
(also in Zone 2) is more expensive at £20.65.

However, for my current set of journeys I needed a Standard Single from
Cambridge. This is because an Open Return is the same price as two
Standard Singles and I can get cheaper singles to go home (e.g. a Cheap
Day Single from King's Cross with Network Card discount and also without
evening peak restrictions). However, a Standard Single from Cambridge to
King's Cross is £17.90 while one to Underground Zone 2 is £20.60. That
difference of £2.70 is more than the £2 Oyster tube fare! When I then
discovered, however, is that a Standard Single from Cambridge to Putney
is only £20.40. As I was going to a meeting near Waterloo it even
included my tube fare there while a Zone 2 single wouldn't have.

What it means is that I have to keep buying tickets all over the place
when I was able to buy everything in one place for the whole set of
journeys in NSE days. Weird.

--
Colin Rosenstiel