This had me stumped for ages, was just waiting at the bit where it waits for u-boot.
I finally found a post that mentioned to try disabling all network interfaces on the PC except the one connected to the same LAN as the NAS. That sorted it for me - I had another active interface for VirtualBox host-only network, disabled this, restarted fvdw-sl-console and everything was fine.
v17 installed onto a new 1TB disk :applause
Thanks!