I think, thanks to a suggestion in another forum - I have nailed this..

Almost every other day, when the set was switched on, it was suggesting
it needed a retune because new channels had appeared or channel numbers
had changed.

Soon after I bought the set, a problem occurred with the software, in
as much as the ability to access directly, programs which had been
shown earlier, disappeared - everyone was affected by the issue. I
found I could improve this a little, by installing a variable antenna
attenuator.

My local transmitter is Emley Moor, but I can also receive a Tyne Tees
transmitter of the rear of the antenna. Once the software issue was
fixed with an upgrade, I was able to remove the attenuator. Any attempt
to retune would then require me to choose which of the two transmitters
to use.

Since when the set has been troubled by regular requests to be retuned.
In another forum, it was suggested that the weaker transmission might
be varying, such that the channels appear and disappear, each
reappearance prompting a retune.

I have reinstalled the attenuator, tweaked down so the set only sees
the Emley Moor transmitter. So far, it would seem to have fixed the
problem.

Maybe someone else is similarly affected?