There was also a famous Cambridge five.
I can comment only on Mare's Nest, and the book is not bad, but as he did not contact any Poles engaged in spying of the weapons (living in England at the time), the overall view is seriously distorted. In my opinion, this has nothing to Irving's political views, just quality of his work. That said, I have seen several worser books, even nonsenses written by renowned professors, so the scientific title means nothing for me, at least in regard of history.
The real problem with Irving is that he has chosen wrong socialist to defend. There are hundreds on similar books about communism, but they do not cause any concern, being politically correct.