Looks like a botched firmware update has bricked my ADSL modem. I'd have thought that manufacturers would learn to keep a backup firmware image in ROM for disaster recovery...
The reason I tried the update is because we've been having major problems with the connection dropping out after only a minute or two at a time. I dread calling support so I wanted to try an upgrade beforehand.
Being the careful guy that I am, I had borrowed our old ADSL2 modem from work (we'd upgraded to VDSL2) and set that up beforehand. Not only as a backup, but another thing to throw at Telecom in case it proved the problem was at their end. It seems to be working without any issues so far so I might throw the boss a few dollars and keep it.