Third day of trying to register an App name for the Windows 8 Store:
Is anyone else out there having any luck?
No idea what made me try this but I copied the URL into another browser and this time, instead of going to the name your App page I was told to enter a security number that had been sent to my phone. This I did and I was subsequently able to Name my App.
So it looks to have been some kind of security issue.
NB I generally use IE (10) but flipped over to Chrome for the resolution.