sternone, I agree with you that a PPL, without any type-specific training would not be able to land a 747 successfully although I'd certainly try it if nobody better qualified would be on board after the pilots had become incapacitated. But that's not what the original question was about. That was about whether it would be legal to fly it, if properly trained (e.g. type-rated) on just a PPL, or whether a CPL/ATPL was required. And the answer seems to be "yes" although the type rating courses are currently aimed at ATPLs, and are taught to ATPL standards.
And no access to a 747 cockpit in-flight... I don't believe it. The cabin is stocked with rescue equipment such as axes and things, and I'm pretty sure that I can get into the cockpit with those tools after a few minutes of dedicated effort. And that's exactly how they should have been designed. After all, the point of the armored doors is not to block access completely (that would require a vault door), but to prevent hijackers gaining access *quickly*. So that the crew has time to send out a mayday signal, set the transponders, make its way to the nearest airport, dump fuel and do all the other things that greatly reduces the hijackers ability to carry out whatever their intentions were.
One of the main 9/11 problems was that there was no warning whatsoever. The planes just dropped off the scopes and nobody knew what had happened until some 90 minutes later. If the crews would only have a few additional seconds in which to send out the "mayday we're being hijacked" then the situation would have been much clearer from the start, and a much better response would have been possible.
How long does it take for a fighter on standby to scramble, take-off, head in the general direction of the airliner and find it on primary radar? That's the time the cockpit doors need to withstand an attack.