Kevin Bean wrote:

I regularly use pre-pay via Marylebone, and only get charged for
a single journey.

This is different from some/most other stations. Beware if you
want to use toilets which are 'outside' the ticket barriers !

I guess you'll be charged a single trip from zone D (Amersham)
to zone 2 (Queens Park).

Marylebone NR to Marylebone LU is an official interchange, and thus the
15 minute Oyster timer applies. You will not be additionally charged if
you go through the barriers from the Chiltern platforms to the LU
platforms.