Ronnie Clark wrote:

It is sad, but understandable, that the BBC did felt it was a better
used of
resources to tape over old programs in the 1970s. This was when most of
the
original video tapes of Doctor Who from 1963-1972 were junked (though
in
at least two instances, this was in error).

What errors were these?

1966's "The Tenth Planet" episode 4, featuring the first regeneration, was
kept in the archive along with the other three episodes of the story. It
only went missing after Blue Peter borrowed the tape to use the
regeneration
clip.

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).

The Tenth Planet 4 was *never* held by the Film Library (although for some
reason they accumulated copies of the other three episodes at various
points). So there was no copy for Blue Peter to lose - they got theirs from
Enterprises who were still selling the story the next year. The Film Library
did however have a copy of The Daleks' Master Plan 4 which was loaned to
Blue Peter and never returned. But this wasn't really "error" and it
certainly wasn't the original videotape.

And 1974's "Invasion of the Dinosaurs" episode 1 only went missing because
it was mistaken for 1968/9's "Invasion" episode 1. This was a moderately
easy mistake to make because, to keep the dinosaurs a secret, "Invasion of
the Dinosaurs" episode 1 was simply captioned as "Invasion" on screen. Ah
well.

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.

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.

At least
one episode, 1965's "The Feast Of Steven", the only episode except for
2005's "Christmas Invasion" to premiere on Christmas Day, was never
transferred to film for overseas sale, and is therefore the most likely
candidate for "episode that will never be recovered".

Mind you from recollection the second series of Dad's Army was not
transferred either and yet two of the missing episodes turned up a few
years
ago.

Well, it's quite surprising just what can show up at times... "The Feast
Of
Steven" was an episode of the 12 part epic "The Daleks' Masterplan".
Because
it was so lengthy, only one copy (IIRC) of the 11 part version (sans "The
Feast Of Steven") was made... Yet amazingly enough, three episodes from
the
serial have now been returned!

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.