Success and a bit of a breakthrough. I formatted the 2TB drive, fixdisk’d it and was able to copy all files across. But when I went to the Portal the box crashed. After trying a couple more times, with the unit crashing again, eventually I was able to get to iPlayer. But then I got a repeat of previous behaviour: I got right to the point of streaming and then the box froze. I tried a couple more times - same thing. I then deleted the streamer_down_file and everything is rock stable again: entering the Portal and streaming iPlayer programmes is problem free. This proves beyond doubt that deletion of the streamer_down_file has solved my problem.
Many thanks MymsMan!