A couple of other probably daft thoughts:

1) is the TV deciding to do a check for software updates at that time? It may do his as an over-the-air update via a particular channel.

2) is some other device (eg. the Freeview box) triggering a change of input source?

MACHINES ARE TAKING OVER THE WORLD  :'(

but .......

I still control the switch on the socket 

Quote from: class37025 on November 07, 2019, 06:10:11 PM
MACHINES ARE TAKING OVER THE WORLD  :'(

but .......

I still control the switch on the socket 

and pay the electricity bill.

If you go to the settings menu and go down to the bottom, you get the option to reset the TV, just in case your mum has accidentally changed some obscure setting. I couldn't find one that switched to HD only.

Thanks to all who have replied so far. It's the switch to HD Friday night and back again Monday morning that tends to mock most answers. My Mum is no good with tech stuff (and I'm not much better) but I can't believe she regularly does something wrong each weekend.
Next time I'm round there I'll have a look at the reset.

Quote from: Newportnobby on November 07, 2019, 09:25:09 PM

Next time I'm round there I'll have a look at the reset.

On a Friday night? 

Does your Mum have a "favourites" list?
My mother-in-law did and some were HD and some weren't. Quite often when we visited we would find that she was watching a channel in none-HD that was readily available in HD. What she tended to do was select one of her "favourites" and when that programme had finished she would often channel surf from that point.  Because Freeview's none-HD channels start at 1 and their HD channels start at 101 she often just stayed within one of these bands until pressing another one of her "favourites" that took her to the other band.
Could your Mum regularly watch an HD channel on a Friday night from her "favourites" list and then watch a none-HD channel from her "favourites" list on a Monday? Hope this makes sense.

Quote from: swisstrains on November 08, 2019, 09:58:58 AM
Does your Mum have a "favourites" list?

Sorry, but the simple answer to that is 'No'
She doesn't watch anything in HD as she says she can't see the difference and her Freeview box is non HD. TBH, I think the only reason she bought the damned telly was because it fitted her TV cabinet (she doesn't like TV's to be 'on show')

It does sound like a transmitter issue unless something locally is being switched on that blocks the 'regular' signal somehow.
You may have already tried this but if not https://www.samsung.com/levant/support/tv-audio-video/what-to-do-if-some-tv-channels-are-missing/

Completely OT but I recall my old dad one night moaning he couldn't get the TV to change channel.

I helped him by suggesting he should try the TV's remote rather than his mobile phone.

Success!

Hope you get the mystery sorted, Mick.

Dave G