Richard J. wrote:

Yes, of course! Now you explain it like that, it seems obvious. Please
ignore my previous posts in this thread. I should have known better than
to try to correct Barry Salter on ticketing!

Thats alright, I was about to point out what my job is... but I shan't
bother now

Just to clarify for anyone who is still confused (there's a lot of
different theories bouncing around this thread) if you have a season
ticket on oyster, lets use this example of a 1-3 ticket, and you have
pre-pay to cover any trips you make beyond zone 3, your extension will
be calculated from zone 4 outwards, as you are covered up to and
including all stations within zone 3.

So travelling Stratford to Heathrow, you are covered for zones 1-3, so
when you leave the network at Heathrow (making sure you touch out of
course) you will be charged for the extra zones you have travelled
through, which are zones 4, 5, and 6.