Just to close this off, and maybe help anyone else with similar problems, it turns out my PRS-T1 was faulty. I tried the hard reset, which certainly improved things, but I was still getting occasional freezes and needing soft resets. So it went back under warranty and I'm told they replaced the motherboard. It's behaved itself since I got it back (I do now have the dreaded Error -9999 when trying to log into the Sony Store but others get that too so I guess that isn't because of my device - doesn't bother me greatly because I don't buy books, I borrow from the library).
So, overall, I'm pleased that finally it seems to have been resolved but pretty pigged-off with Sony's QA for sending out a device that must have been faulty from the start, resulting in a pretty frustrating user experience.