I doubt whether the fact that it might have
brought us into trouble with the Official
Secrets Act weighed with us to anything
approaching the same extent. It was much more a
matter of not 'breaking ranks' or 'letting the
side down'. Moreover, we knew that a history of
the wartime effort was being written at
Bletchley with the hope that this should
constitute an impartial record that might be
published at some future date: until then none
of us should speak.There were in fact occasional breaches, but
these were few, and not at the privileged level
to which the preceding paragraph refers --
apart, of course, from
[Kim]
Philby and his associates. 
The
one instance that I myself encountered arose
when the author David Irving was writing
his book on the German V2 effort and our
attempts to frustrate it, The
Mare's Nest. By talking to a former NCO who
had been at Bletchley
[a Sergeant
Harcourt], Irving discovered that the
reason why we knew the works numbers of the
spent V2s that were being returned to
Peenemünde from Poland was that we had
broken the line of Enigma traffic in which these
numbers were reported.
When GGHQ
[Government
Communications HQ, successor to GC&CS: the
equivalent of the NSA] learned of the
leak, two of its officers called on me and told
me that it would be difficult for them to
proceed against Irving if he insisted on
publishing the text. They had come to me because
they believed that he respected me, and they
thought that there might be a chance that if I
were to talk to him he might accept my
persuasion not to publish. And so his book
appeared without the vital disclosure. Years
afterwards GCHQ told me that they could not have
risked the publicity caused by a prosecution
that would have blown the very secret of Ultra
that they were still trying to keep, and that
this was why they had sought my help.
Unfortunately I did not see much of Irving
for many years following that episode because
shortly afterwards he adopted the theory of
[Rolf]
Hochhuth, that Churchill had
organized the murder of the Polish leader,
General
Sikorski. However, in about 1980 we met
accidentally when he saw me trying to find a
taxi in South Audley Street to take me to
Heathrow. He was driving a Rolls Royce and
promptly offered to take me the whole way. The
drive was memorable for two things.
The first was Irving's driving technique,
which reminded me of Lord Wavell's advice
on handling an army:
The relation between a general and
his troops is very likc that between a rider
and his horse. The horse must be controlled
and disciplined: he should be 'cared for in
the stable as though he were worth £500,
and ridden in the field as though he were not
worth half-a-crown.'
Irving evidently regarded his Rolls in the
same light; 'Everyone gives way to a Rolls,' he
commented as we charged headlong from Mount
Street transversely into the dense traffic in
Park Lane.
On
the way I asked him, 'What stopped you
publishing the fact that we had broken Enigma
when I asked you?' His terse reply was
'Patriotism!'
I tell the story because Irving has at times
provoked many of us; but when it is remembered
what a 'scoop' he sacrificed, surely one of the
biggest ever, he has a lasting right to our
respect alongside Governor Dewey, who in
1944 and in similar circumstances sacrificed his
chance of becoming President of the United
States.
I encountered one other instance where the
Ultra secret was 'blown' if anyone was alert
enough to recognize it. It was in Air
Vice-Marshal Peter Wykeham's book Fighter
Command, published in 1960. In describing
the KNICKEBEIN episode he
wrote, 'By a stroke of luck, Intelligence …
[etc]
By
R V Jones,
author of Most Secret War,
David
Irving recalls: 
write to David
Irving