On Mar 9, 10:50*am, martin wrote:
...which, knowing this group, will result in a particularly
comprehensive answer

I'm planning to travel on a Friday or Saturday evening from Shepherds
Bush to Bletchley, then returning to Euston and back home on the tube,
later that night.

I've got a 16-25 Railcard, and intend to get a Z1-6 off-peak one day
travelcard, and then an off-peak day return to Bletchley. But should
this ticket be from Harrow & Wealdstone (the last calling point), or
Hatch End (the last station in the zones), or is a 'Boundary Zone 6'
available when travelling out of the zones?

The ticket should be from BZ6 to Bletchley. Zone extension tickets run
*from* BZn *to* $station_outside_the_zones only (and back, if it's a
return ticket). As it's a zone extension ticket, not just a
combination of tickets, you can catch any LM train to Bletchley with
that ticket, whether or not it stops at Harrow or Hatch End.

[begin semi-related information that isn't directly related to your
query]

If you were going the other way, for which BZn tickets are not issued,
life would become more complicated, as you'd need to buy combinations
not extensions. Around London a combination of two tickets can only be
used at non-stopping stations if one is a season ticket (which
includes a period Travelcard but not a one-day Travelcard) and one is
not. If both are day tickets, then you need to stop at the changeover
station.

So in that case you'd need to *either* buy a ticket from Bletchley to
Hatch End and change at Watford for a slow train stopping at Hatch
End, *or* buy a more expensive ticket from Bletchley to Harrow and
take that train. If there happened to be a LM train ex-Bletchley that
non-stopped between Watford and London, you couldn't make any use of
your one-day Travelcard at all on it.

If you have a period travelcard, you can do more or less whatever you
like, except for travelling on Virgin.

[end semirelated information]

--
John Band
john at johnband dot org
www.johnband.org