Believability is not the proper litmus test. If it were, all novels would be true, since they are usually believable. In this particular case the former MI6 officer claims to have covert knowledge of Putin's intimate discussions right inside the Kremlin. How could a former intelligence officer, with no organizational support, possibly have access to Putin's private conversations? The Russians are professionals who could be expected to use sophisticated surveillance countermeasures. It's impossible. This guy is flat out lying. Which means his claims of having infiltrated senior levels of the Russian FSB, are almost certainly lies as well. It is difficult task to pull off, even for an intelligence agency with nearly unlimited manpower and resources. All we are really looking at are a bunch of unverified, and unverifiable rumours. This story should never have never made it into any self respecting media outlet.