Rather than using the Virgin box as a hub to connect your NAS, I'd suggest a dedicated 4-port or 8-port gigabit hub, and possibly a separate wireless hub if you are struggling with the Superhub's mediocre signal strength.
If I'm understanding correctly then that does seem to be the solution a lot of people go with. Basically forget about the wireless aspect of the Superhub, it can be turned off, and then it becomes a single port wired LAN modem, and the reported speeds are impressive. Though one of the attractions of the hub is that it should be able to do away with that extra bit of kit, hopefully the new upgrades will achieve that. In the meantime I have various wireless solutions, it's weird though as the Virgin Superhub is made by Netgear and I have other wireless gear from them that pumps out much better coverage....guess they were hitting a pricepoint or Virgin used some proprietary SW that slowed things.
One thing I should check though, all of the LAN cables I have, apart from the Netgear one, are very old, as in at least 7-8 years.
Regardless you've all been a great help and I have a much better idea of what I can look at and the sorts of questions I need to ask when making my buying decision. The Virgin hub is relatively unimportant, it works well as a wired modem so will always be sufficient if I tack on a wireless solution.
Good point V on having spare drives to guard against future failures and unavailability, I hadn't thought of that one.