Ronnie Clark wrote:

1966's "The Tenth Planet" episode 4

Complete myth. The BBC did not have an "archive" until 1978. Before then
its
collection was spread between the Film Library, the Engineering
Department
(videotapes) and Enterprises (overseas sales).

And 1974's "Invasion of the Dinosaurs" episode 1

No, this is another myth. The Invasion 1 had already been wiped. And
tapes
were not wiped on the basis on onscreen title - the label would have
shown
the full title, and it would have also been clear the tape was in colour.
The wiping appears to have been completely in order.

Well... These are both stories from the great Levine, so I should have
guessed that something wasn't right about them

Both (along with others) have been well and truly debunked many times.

It was the very last (chronologically) episode to be junked, therefore
making series 12 (Tom Baker's first series) the earliest series not to
suffer from junkings.

Not true - the first episode of the following story (Death to the Daleks)
was also wiped, but later copies were returned - first a 525 NTSC copy
from
Canada and then a 625 PAL copy from Dubai.

I understood that "Death To The Daleks" 1 was 'mislaid' by Enterprises
(who
to this day, as Worlwide, still haven't 'found' their copy) rather than
wiped. Is this not true?

Curiously Enterprises were selling the Pertwee episodes in both PAL and NTSC
format in 1974, but four years later they never returned any videotapes. So
effectively *all* the Pertwee videotapes were "mislaid". About half of the
originals were still in the Engineering Department in 1978 when a change of
policy stopped the wipings and created the Archive. Death 1 was the latest
one missing.

Roughly speaking the sources for Doctor Who episodes currently existing (and
a lot of other shows as well) are as follows:

From within the BBC:

* Original PAL colour videotapes from the Engineering Department that had
not yet been wiped. (All the b/w videotapes were wiped by 1975.)
* Original film prints for the handful of stories either transmitted from
35mm telerecordings or shot entirely on film, that had been retained by the
BBC Film Library.
* Internal 16 mm film recording viewing prints for odd black and white
episodes that wound up in the Film Library.
* Black & White 16 mm film recordings made for overseas sales (of both the
b/w and colour years) that had not yet been junked by BBC Enterprises.

From outside:
* Black & white film recordings returned from a mixture of overseas
television stations, private collectors, engineers, found in BBC cupboards
and other places.
* PAL colour videotapes returned from overseas broadcasters in the same
format as the original.
* NTSC conversions from the US and Canada. A process called "Reverse
Standards Conversion" has been developed to restore the episodes to their
original quality
* Off air NTSC colour videotapes of broadcasts in the US and Canada. A lot
have been combined with the black and white prints to restore the colour.
* Plus a black & white PAL broadcast quality videotape that was combined
with the colour signal from an NTSC conversion.

I think more than one - BBC Enterprises would make master film negatives
(1
copy) and strike positives as and when needed - at least one set was made
to
send viewing prints to Australia (who declined to purchase). It's
possible
other copies were made for various reasons - the copy of episode 4 that
wound up in the Film Library was probably a print made for internal
reference for one BBC department or another.

It is probably the single positive I'm thinking of. I till find it hard to
believe that supposedly only one positive was made, though. A story with
"Daleks" in the title in 1965/1966?! In between the two Dalek motion
pictures?! Ah well.

Yes but was Dalekmania a phenomenon outside the UK at that time?