Well, that was an interesting day. (Ok, that may be an exaggeration).
Firstly, GF removed the TV and bracket from the wall. Then filled the holes left and painted the wall. All was well. The bracket ws repositioned and marked... and the Gorse Fox headed out to B&Q, Travis Perkins, Toolworks, Covers and Screwfix. Eventually he had some replaced coach screws and suitable wall plugs to remount the bracket.
Having done it once, GF found it easy to re-fix it to the wall in the new position. In fact it was quicker to refit than it was to find the various fixings from local hardware stores. In no time the TV was fixed, switched on and working.
GF then started to set up the Pi to play the DVDs, but first he thought he'd try connecting the USB hard drive to see if it would be powered by the TV. It was. Then, on the off chance, he tried the standard features of the TV and realised it would happily play the DVDs directly from the USB drive. The Raspberry Pi was actually needed. Ten minutes later it was all fixed (with Velcro) to the back of the TV... and GF was playing the Cream reunion at the Royal Albert Hall DVD - perfect.
During the afternoon he set up the other smart features (iPlayer, 4OD, 5OD, Amazon Prime, etc.) and also noticed that the TV had located his NAS devices (without being told) and GF assumes he could view his photos directly from that, if wanted.
Chuffed... and still have a Pi that I can re-commission for some other project.