I feel like a real idiot.
I've been trying to work out why my checkout page (WPEC) has stopped working in Chrome. Firefox and IE are ok.
Turns out to be a probem with this plugin. During my fiddling about, I applied https to my login page. Now I can't log in as admin!!!! Not in any browser. I can only login as one of my test customer profiles I created, which are not admins!
What the hell have I done?