The approach I used yesterday seems to have the phone in what I would call a "normal" state (ie: it no longer drains to 0% after 2 hours on standy).
1). shut down all apps ie:
- double-click the home button to reveal the running apps
- select a running app by holding your finger on it's icon until they all start to "shake"
- shut each one down by touching the red "-" until there are no more icons in the task bar
2). Reset the network (which forces a reboot)
- from the Settings "apps" select: General -> Reset -> Reset Network Settings
- this should force a restart of the device
Unfortunately i did all of these at once so I'm not sure which resolved my problem, but I now have something approaching a usable phone.
FWIW also upgraded my iPad Mini to 6.1 (glutton for punishment I know) and it's been fine (again, I had to shut down all apps as some kept crashing on startup until I killed off everything)
YMMV