Andrea wrote:

Bob Wood's reply was essentially correct, but his answer to your final
question was a little unclear so I'm going to reply myself separately.

My local underground station (Holland Park) didn't have the info to
hand, so can somebody kindly tell me which National Rail/tram routes
in London my annual Z1-3 Oystercard can be used on?


You are perhaps suffering a bit of understandable confusion over
terminology here Andrea. What you have purchased is a zones 1-3
Travelcard, which has been loaded onto your Oyster card - so you have
a Travelcard on Oyster. This offers exactly the same validity as a
paper Travelcard.

Your Z1-3 Travelcard is valid for unlimited travel on National Rail,
as well as the Underground, in zones 1-3. In addition Croydon Tramlink
accepts all Travelcards that are cover any of zones 3,4,5 or 6.

Most National Rail routes do not accept Oyster Pay-as-you-go (PAYG) -
but as you have a Travelcard on your Oyster card this is not relevent,
at least not for journeys wholly within zones 1-3.


Is it also true that I can use any bus service in greater London,
despite only being able to go as far as Z3 on the tube?


Yes. This is a fairly recent change (within the last 5 years or so),
but now all Travelcards, irrespective of what zones they cover, are
valid on all London Buses including night buses in Greater London.
They are also valid on some bus routes that cross the Greater London
boundary and serve areas just beyond.


If I wanted to go to a station in Zone 4, do I have to buy a single
zone extension prior to travel? How much does that cost, e.g. Z3 to
Z4, or Z2 to Z3?


Depends if you're travelling on the Underground/DLR or National Rail.

If you're going by Underground/DLR, then you use Pay-as-you-go to
cover the cost of the ticket extension. You do this with the same
Oyster card that holds your Travelcard - i.e. you are combining the
Travelcard function and the Pay-as-you-go function of your Oyster
card, and you will only be charged for the fare from where your
Travelcard validity ends.

To take advantage of this you'll need to add some Pay-as-you-go credit
to your Oyster card - just go to the ticket office, ticket machine or
one of the many newsagents who top up Oyster cards and add some money
to your Oyster card there. This balance doesn't expire.

If you wish to travel outside of your Travelcard's zonal validity on
National Rail, then you will need to buy a paper ticket extension from
a ticket office before you travel.

There are a very limited number of National Rail routes where you can
use Oyster PAYG - for example on Chiltern from Marylebone to Harrow-on-
the-Hill - see this link for a list of these routes:
http://snipurl.com/Oyster_PAYG_on_NR

The number of routes where you can use PAYG on National Rail is set to
expand in the future, which is good news but as it's going to happen
bit by bit it will cause enormous confusion!