I had the flashing red LED and no charging problem.

I tried both my 3rd party N900 USB cable and also the small CA-146C adapter which lets you charge N900 via the microUSB by adapting a standard Nokia charger.

Neither worked as I had depleted the battery too much and apparently neither supported the emergency charging mode needed in this case.

However, I dug out my genuine Nokia USB cable and bingo! The battery I was about to throw away as a dud is now charging fine.