So, if you connect it via USB it’s supposed to show as a drive in windows, yes?
I plug mine in and get the lovely dancing rainbow of lights, but the OS has no idea anything is there…
Also, it isn’t visible on mbed.org as a platform is there another way to program it?
Welcome to the forum!
I ran into the same thing:
https://www.ghielectronics.com/community/forum/topic?id=16227&page=4#msg162305
Long story short: hold down the tiny reset button
For programming see this post:
https://www.ghielectronics.com/community/forum/topic?id=15929&page=6#msg160859
Long story short: They (mBed) are working on an issue regarding this, so for the time being use the LPC11U24.
Glad it worked. But there’s only 1 superhuman around here … and that’s where I got my help from! 
(see the poster’s forum ranking who provided the info in the 2nd link )
With the battery is it safe to plug into USB power? Or do I need to remove the coin battery when plugged in?
@ Dr9 - It is safe to leave battery in. There is a diode on there for protection.