I have a 64bit host, but can't install 64bit guests

[seanbw]

I am using the latest version of Virtualbox and so far I though everything was OK until I wanted to install Ubuntu 16.04 in a virtual host when I realised that all the virtual hosts I have been creating are all 32bit.
I read somewhere that it could be because virtualisation had not been activated.
I checked my BIOS and virtualization had been activated in Bios.
My PC is a HP Gen Z800 server/workstation with 16GB RAM, 2 TB Hard disk, 8TB of removable media, Quadro 600 Graphics card, Xeon 5520 Chips x 2
The VT-X has been activated as the system report shows below.
Security\System Security

• Data Execution Prevention: Enable
  Virtualization Technology (VTx): Enable
  Virtualization Technology Directed I/O (VT-d2): Enable
  AES Instructions: Enable

I tried to install Ubuntu 64 bit but the only option I had was a 32bit Ubuntu.
What else can I do to change this option as it has so far prevented me from using Virtualbox.

[Perryg]

FAQ: I have a 64bit host, but can't install 64bit guests.