…and I’m back online 
After my failed attempt to update the firmware of my new Huawei E5830 — causing it to die a quite nasty premature death — I took it back this morning to try and get it exchanged. The branch of Carphone Warehouse I bought the first one from didn’t have any left so I had to make a special trip to Kilburn High Rd — a beautiful place and well worth a visit. The guy who served me was very helpful and got me sorted out in just a few minutes. Like me, he was baffled as to why the original modem should suddenly stop working like that — strange.
Lesson learned, I am now back home and writing this on my personal Mi-Fi connection. I’ll leave well alone this time (well that’s the plan at the moment anyway).
the modem should have been upgraded on windows xp machine. the firmware update works on that windows only.
if you have the defected hardware try to reupgrade on an xp machine
Ah, that would explain it (I used Windows 7). Shame the how-to didn’t mention that. Thanks for pointing it out, although it’s water under the bridge now — I got it replaced.