Ronnie Clark wrote:

I'm afraid the BBC's own paperwork is fairly conclusive, as well as
interviews with Verity Lambert, the show's first producer. Every episode
of
Doctor Who was pre-recorded to (mostly to video tape) at least a week
before
transmission. It was curious, however, how this progressed in technique
over
time. At the very beginning, episodes were recorded on a one-at-a-time
basis. The recording was made "as live" (with as few breaks in recording
as
possible). Certain scenes were pre-filmed, and fed in where required to
smooth the process (Though the very first episode, "An Unearthly Child",
with what appear to be complex pre-films for its flashbacks were actually
achieved by clever direction meaning the actors who were on one set only
had
to provide voiceovers for the flashbacks), but actual edits were extremely
rare in the first series. This was due to the only way of editing being to
physically cut the 2 inch tape and splice it together. "An Unearthly
Child"
contains one edit approximately half-way through.

There was actually another method of editing available as well. An episode
would be recorded not on videotape but as a 35mm film recording, which could
then be edited by splicing. In the later story "Planet of Giants" two
videotaped episodes were cut down to one for transmission by transferring
them to film and cutting (and the final episode has a lot of scene changes
where the cast have moved - something else that shows up front that the
episode was prerecorded).

It was quite some time before more advanced editing techniques made it
possible to record a whole serial in one go, however I vaguely recall that
it started in the middle of the third series.

They seem to have still been recording one episode at a time right up until
the end of the sixth series, and even in the next few years the doubling up
was mainly to save on the cost of storing sets. From about the seventh
series they seem to have been recorded on a stop start method.

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?

Most episodes from this era have
been restored either from overseas film prints, or in some cases from NTSC
transmission tapes and, even more curiously, from home recordings. 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.

As for the reason why the BBC chose to pre-record Who from the outset: who
knows? As has been pointed out, this was certainly not the norm at the
time.

I was under the impression that by 1963 it *was* the norm for drama shows -
Z-Cars seems to have been regarded as unusual for still being live.