My bargain may have been too good to be true.
All was well yesterday, I'd even tested the built-in BitTorrent client by downloading the latest Raspbian Wheezy image for my Raspberry PI, all went speedily and without hitch.
Left the NAS switched on overnight.
Had things to do this morning, but when I went to look at it around 3pm, I couldn't connect to it.
It's all still powered up, router shows LAN link is up, but web interface not responding, and can't access via Samba.
The metalwork of the USB port at the back of the NAS was unpleasantly hot to touch. (I'm not using the USB)
Power cycled it, but still can't get connection to it.

Will have a proper look at it later.
Ian