My understanding is that all the major WW2 powers were working on nuclear weapons in some capacity and that there was a major British effort in Canada (code named Tube Alloys) that, after the US entry to the war, was folded into the Manhattan Project. My understanding was that there was an agreement of shared technology and research from the joint venture but that the British lost their only copy of that agreement. It was made between Churchill and Roosevelt and Truman did not want to honour the agreement with Attlee.
Is this correct and if so how did this happen?
The US and UK signed the Quebec Agreement in August 1943. From that point onward, they were meant to be equal partners in the Manhattan Project. In practice, the US did try to limit the access of the British to things that they thought the British could definitely contribute to (so the British were never allowed to visit Hanford, for example), but in general there was a lot of information exchanged.
The end of this arrangement was during the Attlee administration, but not because of Attlee himself. It happened in January 1947, when the Atomic Energy Act of 1946 came into effect. The US Congress, in drawing up the AEA (which governed atomic energy matters in the peacetime), essentially decided that this stuff was too dangerous to share and that until the nuclear arms race had bene averted with a treaty (aka, never), it was illegal to share sensitive nuclear weapons information with any nation — even allies. So this cut the UK out completely.
The UK were unhappy with this, understandably. Interestingly, the people in Congress who wrote the law don't seem to have known about the Quebec Agreement. So the Congressmen were a little surprised to learn about this later. The UK response to this was to try and find ways to reestablish the relationship, as well as pursue their own independent nuclear program under the assumption that this would allow them a "seat at the table."
The UK had some leeway at reestablishing the relationship. During the war, the US and UK had set up a joint organization to divide up the supply of uranium that the two controlled. This meant that the US had to basically get favors from the UK if they wanted to accelerate their program dramatically. There were various people within the US nuclear establishment who thought that cutting the British out was unfair and unhelpful, and sort of worked around the edges of the law.
Ultimately there were plans to try and change the law and find a way around it (in part because the US wanted to base nuclear weapons in the UK), which were somewhat thwarted in 1950 when it became clear that the main spy at Los Alamos — Klaus Fuchs — was part of the British delegation, and that the British had a lot of penetration by Soviet agents. Nevertheless, by 1954 the law had been amended in ways that would allow the US to exchange classified information with the UK again, reestablishing the "special relationship."
So anyway, this is a brief overview, but you can see that a) it had nothing to do with Attlee per se, and b) it actually had nothing to do with Truman. It was the US Congress that wrote them out. And it wasn't anything quite as simple as losing a copy of the agreement, but it is the case that the agreement was itself secret.