OK.
I went and got a 2000T 1TB from CEX.
The 2000T wouldn't respond to the remote that came with it: RM-I09U. I tried holding the 0 and OK buttons, and changed the mode from 1 to 2, but it still was weird... it slowly registered me pressing the '0' and '3' buttons, but nothing else, not even the '2' it had allowed earlier. (Batteries were new.)
But the 2000T did seem to respond quickly and fully to my old RM-H06S remote (from the 1800T), and was happy playing the files from my external drive. OK, I thought, I can live with that.
But then I noticed that this 2000T / RM-H06S combination would allow me to change volume on SD channels, but not on HD channels.
For example:
Channel 2 (BBC TWO) responded to the volume button, showed the change via the 0 to 20 graphic, and the sound went quieter or louder.
Channel 102 (BBC TWO HD) responded to the volume button, showed the change via the 0 to 20 graphic, but the sound remained 'loud' - no change discernible.
I would ask for advice on this weirdness, but the refund clock is running down and my knees are complaining after the box swapping!
So I'm taking this 2000T back to the shop for the 48hr no-quibble refund, and will wait for another 1800T.