No I hadn't missed anything but after I changed a couple of BIOS settings (OS and SATA mode) I can now install DSM from Synology Assistant or from the web interface. But then, nothing. Both network ports stop working - green lights only, no orange, and it's just no on the network completely, as if the ethernet ports have died. If I remove the disc and reboot, all back. Wipe the disc, put it back in, re-install, same. I've used several discs and different .pat files.
If I look at the discs, they look like I'd expect, with a partition for DSM and partitions for data.
Completely stuck, can't think of what to try next.