Thanks Guys. Most helpful, and I think I have somewhat of a head start for when the NAS arrives even though it's clear that I won't know for sure what I need to do to make this temporary installation until it does. Stay tuned; I'll report back.
Eventually, when living in the Garage, the NAS will have a gigabit cable connection* to Netgear GS605 switch in my office** and from the switch to one of the two ports in my BT HomeHub in the hall. The switch will also connect to the media server (replacing today's WiFi connection to the HomeHub) and to the living room ready for when I purchase a PS3.
*Why am I not using the Netgear HDX101 Homeplug approach that I already use very happily and very successfully to connect HomeHub to secondary wireless access point upstairs that covers the garden? Because performance from the garage (which is on an electrical spur) via Homeplug is ho-hum for backing up large files. Performaing the same trial file copy ten times between the same two laptops took between 0:55 min and 1:05min entirely via Wifi between office and garage and 5:20min and 7:15min via both WiFi and temporary ethernet from office to HomeHub and then Homeplugs to the other laptop in the garage.
**Why haven't I laid the ethernet cable to the garage already if it's my intention to do so, thus negating the need for today's post as the NAS will be on the "normal" network? Because I want to be sure I won't have fried network cards (or worse) in event of electrical storm if I run cables under ground between two separate buildings with different ground potential. My friend who designs remote networks for a living is visiting at the weekend and already knows I'd like such discussion!