Reading through some of the comments over on /r/history about the transcripts of German scientists on the dropping of the atomic bomb; one of the users remarked about Heisenberg being instrumental in the Germans creating an 'engine' not a 'bomb' and goes on to explain a chain of evidence based on Heisenberg's fluctuating calculations for the necessary materials to create the atomic bomb: is there further evidence to support the suggestion that Heisenberg intentionally sabotaged the atomic bomb from within or is essentially baseless speculation?
I've given answers on this and related questions a few times on here before:
Were Werner Heisenberg's nuclear capabilities overestimated?
And I've talked about this a little on my blog: Historical thoughts on Michael Frayn's Copenhagen
The gist of them all is: no, there isn't evidence that Heisenberg tried to sabotage the work. And even Heisenberg didn't actually claim he had tried to sabotage it — he implied, at most, that their spirit wasn't into making a bomb, but even this isn't really in evidence.
The issue with the German atomic bomb project is that it wasn't a bomb development project. From 1941 onward they focused on making reactors. They had a pretty-good program for that, but it was modest in size and deeply affected by war-related matters (bombing, sabotage, brain-drain, etc.), and as a result didn't get any of them up and running. But they never made a commitment to making weapons, never had the infrastructure in place to do it, and didn't look into the question other than paper estimates.
As for Heisenberg, he was very interested in making reactors. There is a big step from making a prototype reactor to making a bomb. Could Heisenberg have pushed harder for a big bomb program in 1941? Probably. But his lack of a big push is not evidence of sabotage — everything we have suggests that the Germans correctly perceived that making an atomic bomb would be very difficult, if it was possible at all. And they decided to spend their resources (human, material, and financial) elsewhere.
There is really only one author these days who pushes the "Heisenberg sabotaged things" thesis and he is Thomas Powers, a very capable journalist, to be sure, but none of the historians of science who have looked at this in detail have found evidence of sabotage. What they have found is a very deliberate strategy on the part of some of the scientists involved (von Weizsäcker in particular) to imply, in the postwar period, that the German physicists had actually taken a strong moral stand, and indeed were more moral than the Allied scientists in this respect. The self-serving nature of this is fairly obvious.
The professional historians, to be sure, do differ in their specific reads on Heisenberg. Some take the position I have advocated here, that it wasn't some kind of great failing of his to have not built a bomb. Some castigate him for intellectual sloppiness. Some see the failure as an indictment of German science. Some think Heisenberg was very pro-Nazis. Etc. There is a lot of variety. But the one consistent theme is that nobody thinks Heisenberg tried to sabotage anything. There just isn't any evidence of that in particular — and much to the contrary. (Also, all of these narratives probably overstate Heisenberg's own, personal role.)
A great overview of positions on this question can be found in a little volume of papers that came out to accompany Michael Frayn's play Copenhagen: Matthias Dörries, ed., Michael Frayn's Copenhagen in debate: historical essays and documents on the 1941 meeting between Niels Bohr and Werner Heisenberg (Berkeley, CA: Office for History of Science and Technology, University of California, Berkeley, 2005). (Full disclosure: I designed the cover!) Separately one can look at some of the fine books by historians of physics on this subject, including but not limited to Mark Walker's Nazi Science (which is the book I usually recommend to a non-specialist audience, as it covers a lot of ground very clearly and soberly), David Cassidy's Uncertainty, Cathryn Carson's Heisenberg in the Atomic Age, Paul Lawrence Rose's Heisenberg and the Nazi Atomic Bomb Project, 1939-1945, and, if one is really curious about dissecting the Farm Hall transcripts, Jeremy Bernstein's Hitler's Uranium Club (which is a heavily-annotated version of them by a physicist/historian).