Well I managed to 'recover' it. If you count losing all my data (luckily there wasn't much) as a recovery. This should never have happened in the first place and frankly it's shoddy that it did. All I did between my phone working, and not working, is let it run out of batteries. Does anyone know if this is an android problem or specific to the Pulse?
It's obviously quite common given that about four people have asked questions about it, and the pulse has only been around for about a month.
Anyway, here's my coherent guide to getting things back:
1. If you can't boot your device then the only way to get into the recovery screen is to download the recovery image (this is a small boot-loader type piece of software) via fastboot and run it directly. I.e. there's no point actually flashing it to your phone.
To do this, download the Amon-RA recovery image in the other thread and run ./fastboot-whatever boot the_recovery_image_you_just_downloaded.img. Obviously you have to have connected your phone in fastboot mode. I could only get this to work in linux - didn't work in windows for some reason.
2. In the recovery mode. First select Wipe, (the factory reset one). It will erase all of your data.
3. Next select the enable USB mass storage mode.
4. Now copy the signed-update-pulse-whatever-its-called.zip file to the SD card (which should have appeared as a USB drive on your computer).
5. Finally select update (choose zip) on your phone and select the zip file you just copied.
Your phone should now be as you bought it, without root either.
Grrr. On the plus side I now understand how android works much better...