I finally got hold of ebay customer support on the phone after waiting for awhile to see if the shop would come back. Then got transferred to another person as the first person couldn't help.
The second lady told me there had been some updates that she couldn't tell me about (!) and that had affected some accounts that had a history of frequently changing their store subscription level. She pointed out that I changed my subscription level in 2012! (sounds frequent to me for sure). The fix was to reopen the shop. Which I did. Which didn't work. After that, she said she would get a report to technical and would get back to me within, hopefully, 24 hours. That was about 40 mins on the phone.
An hour or so later, the shop was working again and I had an email from her that said she had 'refreshed' my account on her end and it was all working.
So if this happens to you, make sure you call them. It clearly isn't going to fix itself.
