Let's suppose that we weren't in a pre-election phase, and that it was alleged that a minister - not the Prime Minister, just any old minister - had been alleged to bully his staff so severely that he had to be dressed down by the Cabinet Secretary and those staff were driven to calling an anti-bullying charity. Suppose then that the minister involved stated that these allegations were "lies" and that his political allies went all over the press saying that these were manufactured allegations, that the book was a work of fiction, that the allegations were generated by the minister's political enemies and politically motivated. Then suppose that an anti-bullying charity confirmed that in fact they had been contacted as alleged - indeed on multiple occasions.
Wouldn't that be a bit more than "politically damaging"? If it were anyone other than the prime minister, and anytime other than pre-election, mightn't it actually be a potential resignation matter?