I have taken a B747-400 up to FL430, just once, as it is very rare to have the circumstances condusive to enable it. Indeed, I don't recall going higher than FL400 or maybe FL410 ever before this one off.
It was coming back into LHR from JFK in 2009 on exiting the NAT system over Southern Ireland and they even coordinated a handover to London at that level rather than the usual FL330(?).
Anyway, I have the photo to prove it somewhere!