I have now managed to work out a way, though I am not entirely sure how I managed it !!😳
I had done a factory reset before I got started, however, I tried the short RF lead idea, but that just got me no additional or ANY TV channels to add to my setup. So, I did another factory reset before I got started again, and so I had no channels on my system setup (not showing), so I tried to do what others have said, and select Digital Guide before I chose Initial Setup, which I have done in the past, and then chose the usual options, English, UK, Search for stations, which, as usual found the correct number of channels, with a few duplicates which it deleted and completed setup.
Then ... the same'ol..same'ol situation, select a TV channel, if you can, as it takes ages to move from one TV channel to another, though you can go from 1 (BBC1), to 2 (BBC2), then 3 and 4 and 5 and 6 are missing, so it eventually tries to load channel 7, each channel displays a channel screen, current programme, for about 3 to 4 seconds then the screen image 'freezes' and goes 'blank' to black. Other channels are missing or work as the others I've mentioned. The TV Digital Guide screen is more of a lilac, and yellow theme colour scheme than the usual blue, and many of the TV programme stations are missing from the schedule. This is the issue I reported from about 24 August 2022.
So, I did I another factory reset before I got started again, and so I had no channels listed, so I opted for Digital Guide once more, but then I put the ususal RF/aerial lead in, and of course it showed I had no channels to display, so I went to Initial Setup and chose the usual options, English, UK, and set another few settings that I usually choose, but did not do a TV channel/station search, I just exited back to what would usually be where the TV channels would display, to what I had expected would be a 'blank' black screen, but then I got BBC1 displaying working as usual - even though I had not done a search of TV stations/channels at that point - so that was weird in itself. Several other channels were in their usual place, but ITV, Channel 4, and Channel 5 to name just 3 TV channels, were listed in the 800s section, so I went into channel sort and moved them to where they should be, and then I exited that part, and went to Digital Guide to see what that then looked like. So, I had BBC1 as 1, BBC2 as 2, ITV1 as 808, and Channel 4 as another 800 channel, the same for Channel 5, but they were listed and they worked, but the Digital Guide still had a lilac/yellow colour scheme.
I switched the system off at that, happier that I had most, if not all, of my TV channels working again on the RDR-HXD870, even though they were in sort of weird listing/numbering order and the colour scheme was unusual.
Later that day I switched the system back on and the usual blue Digital Guide screen was present, and the lilac/yellow scheme was gone.
I do not have an ideal setp, and it is not working as it should be expected to work, but it is working, so currently I am grateful for that.
Thanks to all for your suggestions, ideas, but none of them seemed to work for me, and I am not exactly sure the process was exactly as I have described/explained it, but it is working for the most part, so I can use it.
Having said that, SONY should never have allowed the situation to get to this sorry state of affairs where customers have to 'pull their hair out' to try to solve an issue that should never have needed fixing by many other adversely, and similarly affected customers having to offer their help to other customers, while SONY employees don't interject, even to offer solutions, whether they be hardware, software or guidance, as none of that has been forthcoming from the manufacturer. SHAME on SONY!
