The fall of Singapore

I am still haunted by the BBC programme last Monday on the fall of Singapore to the Japanese in the Second World War and the alleged perfidy of the Lord Sempill.
The programme reported on files recently released into the public domain showing that Sempill and another flying pioneer, Frederick Rutland, were under investigation by MI5 almost from the beginning but didn't act. To those who argue that it is wrong to accuse a man who having died in 1965 cannot answer his critics, the programme showed that he did have the opportunity to make his case in a specially held tribunal and he failed to allay the fears of those who feared that the nobleman was sharing secrets with the Japanese and actively helping them to build a carrier fleet of such power it was subsequently able to carry out the devestating attack on Pearl Harbour.
It can be argued that the Colonel and Master of Sempill was initially helping British allies because during WWI, the Japanese were indeed on the side of the allies but as their imperialist ambitions broadened, so did their need for the latest war technology including carrier know-how and the design of massive airborn bombs. However, such was Sempill's apparent affection for the Japanese, their political philosophy had some resonance with his own right wing ideology, he continued supplying information to them for small payments despite it becoming clear that they were becoming a likely enemy. That he was selling information to a prospective enemy was known to MI5 and eventually Winston Churchill. Sempill even served in Churchill's war office and was was almost certainly the source of top secret information on the meeting of Churchill and Roosevelt that was passed to the Japanese and intercepted by Bletchley Park, the British surveillance centre. Still, nothing was done to curb the man's influence, MI5 fearing that to expose him would compromise the secret's service own information gathering techniques.
Meanwhile, Rutland, a dedicated flier had left the British military and hired himself to the Japanese for the purpose of training their pilots how to land and takeoff from carriers, skills they were to use to devastating effect at Pearl Harbour and in the invasion of Singapore.
Many questions are raised by these revelations, not least by those who lost their loved ones to the Japanese.
Why did MI5 take no further action against Sempill when it became clear he was acting against the interest of the UK? Did they fear that he might betray other, at the time more serious, secrets?
Why didn't Churchill act more decisively knowing he had a possible spy in his midst? Did Sempill have some hold over him? Did Sempill have some knowledge of Churchill's private life? The MI5 might have had him assassinated but didn't. Did they fear a secret being uncovered in the event of his death?
Sempill was interned during the war against the Japanese but was left to live out his life in peace, receiving a distinguished medal from his sponsors.
Rutland took his own life after the war.