Further information:
I tried to install it again and when it stops at the error message I pressed Alt+F1 to open the command line. I typed
lspci - v | grep "Class 0200" -B 1
to get the list of network cards and I got:
Intel Corporation 82574L Gigabit Network Connection [vmnic01] Class 0200: 8086:20d3
I'm wondering if the [vmnic01] part could point to the reason why it's not working? (I'm clutching at straws here).
Also, if I look in the sys log file when I try to load up ESXi (not from the installer) there are some error messages:
jumpstart [2097561]: PortGroup: NIC vmnic0 does not exist
Does that give any clues as to what might be going wrong?