Tim Roll-Pickering wrote:

Does this also mean I can't get a YPR discount on an Amersham to Aylesbury
ticket (a journey I'm due to make several times).

(Indeed what's the cheapest way to get a good discount on an extension from
a 1-6 travelcard to Aylesbury? PAYG to Amersham, then buy a return ticket at
the office there and on the way back jump onto the platform to touch in then
back onto the train, or a straightforward extension from Marylebone/Harrow?)

As has been said above, you'd need to hop off at amersham to complete
your Oyster journey, and buy a return from there to Aylesbury (£7.00
at present).

On the way home you don't need to worry about the gates being closed,
as they are always open if no-one is manning the gateline, and even if
they are closed, there's three remote readers on the southbound
platform - one by the manual gate, one by the bottom of the footbridge,
and one toward the northern end of the platform (by the bike shed).
Just make sure there's enough PAYG to get moving, and touch it once on
the remote reader, and like magic it'll say "enter" and conveniently
remind you of your remaining balance before you leap like a gazelle
back on your train. Or wait 10 minutes for the next met if you're too
slow.