As I mentioned, I'm out and away from my desk - this reply is in reverse order because of what I was thinking at each edit. Sorry about the confusion, just read the whole thing before acting.
When I do so, the Bootloader Mode loads. The triangle with an exclamation mark inside appears when I select the Recovery option from the Bootloader list.
OK fantastic - your bootloader is fine, and that means that the FASTBOOT utility is fine, so you can apply the full RUU if I can find it when I'm back at my desk.
Meanwhile - the utility I'm looking for is going to erase your data and I'm sorry about that but it is what it is.
So it's safe for you to try the factory reset from the bootloader - that's the only factory reset that will try and rebuild the partition tables.
I don't know if that will help but aside from your data, it won't hurt.
If it says FASTBOOT at the top, you get to the reset option from HBOOT - same place that had RECOVERY.
You can also get to FASTBOOT from HBOOT and here's something else to try -
Remove sd card and sim.
Plug into pc so it says FASTBOOT USB at top of phone.
Get the Mini-SDK -
http://androidforums.com/threads/adb-guide-updated-2014-11-11.443072/#post-5389081
Get the sdk-tools.zip unzipped with Windows Explorer and then shift+right_click on the sdk-tools folder - choose Open command window here.
In command window say -
fastboot erase cache
fastboot reboot
I don't think that will help but every once in a while it solves the problem so it's worth trying - and unlike the other things discussed, won't erase your data.