Clive D. W. Feather writes

Hampstead Heath is a tweak. A hack. Every station around it is in zone
two, no matter what direction you go in or how you go there.

Not true. You can draw a reasonable-looking boundary between zones 2
and 3 with no odd loops or "islands".

I'm going to throw bus zones into the equation. OK, there are minor
differences between the rail/bus zones - normally to give advantage to
the passenger. A good example is Finsbury Park; both NR & LU stations
are Z2 only - but the bus station is in Z3 only, as all the routes from
the bus station (210, W3 & W7) head off NW along Stroud Green Road.

Hampstead Heath NR station is well within bus Z2, the Z2/3 boundary is
at Hampstead LU station. Looking at a geographic bus map, a definite
loop would be required to fit Hampstead Heath NR station into Z3 and a
casual observer would doubtless be surprised to find that the station is
in Z3.

Compare and contrast the situation with Upper Holloway NR station. Here
the bus map shows a definite loop of the Z3 boundary to include both
Archway LU and Upper Holloway NR stations - creating an overlap area of
Z2 & Z3. So whilst Archway LU station is in both Z2 & Z3, for some
reason Upper Holloway NR is resolutely Z2 only.

I'm still convinced that the jiggery-pokery with the zone boundaries was
just a cynical ploy to make the NLR franchise (what is now Silverlink)
look more attractive at privatisation, by increasing potential revenue.

http://www.tfl.gov.uk/buses/pdfdocs/n_west.pdf

--
Dave