Netboot cannot get DHCP IP address
There doesn't appear to be a way to confirm that the client is getting a correct DHCP entry at this stage (no logs, etc). Packet sniffing helped.

Basically, the incorrect options in DHCP was preventing communication with the Netboot server, but with no diagnostics on the client, it was fairly hard to troubleshoot.

Managed to tweak infoblox settings correctly - I made OSX server a DHCP server on a closed subnet, and captured the boot options. Once I had those, I ported them to Infoblox and managed to get booting OK