Before I do a thorough process of elimination in an awkward cupboard, has anyone experienced this problem before?
Sounds like an internal failure of the device (it happens) ... call Vodafone and get them to ship you a new one!
Turns out it was the cats fault. He must have played with some LAN cables in a hard to reach spot under a chest of drawers (hence I wasn't keen to troubleshoot there initially)
Sorry VF, I tend to default to blaming the ISP-provided device...