Actually the PAYG fare would be for Zones 3-6, otherwise he would not
have a valid ticket between Northfields and Boston Manor (across the
zone 3/4 boundary). The costs are the same, though.

--
Richard J.
(to e-mail me, swap uk and yon in address)

Actually it would be for 4-6, because his travelcard covers zones 1-3,
and so the extension would be for the additional zones he is travelling
through - zones 4-6. Otherwise he'd be paying twice for zone 3.