Ok, so after over 2 months of this issue on a HP Proliant DL380p Gen8 server, HP support FINALLY was able to find a resolution. And, as I thought, it was a BIOS setting, but a hidden one. To resolve:
1. Boot up the server to the BIOS by pressing F9 at the appropriate time (will see 'press F9 for Setup' displayed)
2. When in the Setup Utility main page, press CTL+A. This will display a new option at the bottom called "Service Options"
3. Arrow down to Service Options, then arrow down to PCI Express 64-bit BAR Support and choose to ENABLE it
4. Press ESC out of the Setup Utility, then press F10 to save and exit
Reboot the Host and you should now be able to install ESXi 6 without the 'Decompressed MD5' error.
Hope that helps someone!
Regards.
@coolsport00