Ok, we managed to get this one resolved ourselves, after a day of research and configuration.
What looks like has happened is what we all probably expected. it's failed during a software update.
The reason we we're getting the battery low symbol was because the battery was low. Due to the software issue, it was very unlikely the phone was being instructed to take charge properly. so it was sat in power limbo.
We managed to get to recovery mode by. volume down + power. then as soon as the image disappears on screen hold volume up, all the way through the buzz and to the Nokia Logo.
Thn you get an image of a cog and lightning bolt. Recovery Mode/Flash mode, whatever you want to call it. Now the hard part, getting a flash tool and a ROM.
After searching numerous sources online we found that we need two tools.
* Navifirm+ 1.7 (To Download ROM/Firmware)
* Nokia_Care_Suite_5.0_2012.45.4.5 (To Flash)
The problem here is that NaviFirm has been blocked so that when it starts up you don't get the list of phones to select ROMs from.
Unless you follow this tutorial we found. [http://www.youtube.com/watch?v=Tk4F792IMWg|Navifirm 2013 100% WORKING!!]
We used the link in that videos description to get NaviFirm, but read the comments if you cant get it working. After you extract the files check the size of the "cache" folder. Now run NaviFirm and wait, you'l get an error. then check the cache folder again. It's now empty!
Just unzip/extract the software to a different location to the original, and copy the contents of the cache folder to the cache folder that's just ben cleared.
then run the original NaviFirm again and hurrah, it works.
In regard to which ROM you need, it depends on your handset/location and carrier ect. For the Lumia the ROMs we're rm-820 and rm-821. then look throgh the lists to get one with a name that looks relevant to your device.
To Flash the software we watched this tutorial as it's relatively straightforward.
[http://www.youtube.com/watch?v=O4tU4oRkyAs|Lumia 920 Flash Tutorial - Brick fix!]
So there you go, even if your 920 is bricked, so long as you can get to the Flash mode screen, you can probably fix it.
Alternatively, it looks like Nokia are aware of this issue and we've found numerous reports of people with this issue having their handset swapped no questions.
We will be posting a more detailed guide to this repair on our website at [http://www.digitalplumbinguk.com|www.digitalplumbinguk.com] If the community here do think this is useful then we'll post it here too but so far this thread hasn't had much activity.