TV auto-tuning to wrong transmitter

But why would the TV select the signal from the furthest away transmitter, irrespective of how strong they are? It's not a fault in the TV, is it?
It's not picking one over the other, it's simply using them as it finds them.
TVs normally scan from low frequencies upwards, and someone else has said that the unwanted transmitter has the BBC mux at a lower frequency than the one you want. So the TV find that one first and tuned it, then later it finds duplicates and has probably stored them up at 800 onwards.

One question for my own education, if I may. The table you've linked shows that CP is 25dB less than SH transmitter. But why is 25dB attenuation appropriate -- don't we want to eliminate the CP signal if possible, i.e. 43dB attenuation? Or is 25dB simply a good compromise, and going further would reduce the SH signal too much?
That's about it. If you attenuate the signal too much then you'll lose the one you want.
EDIT: The idea is to reduce the weaker signal to the point where the tuner no longer picks it up - but without reducing the wanted signal to that point.

There are many variations. If an attenuator works then that's simple (you might only need it when tuning, of if you have a good wanted signal then you can leave it permanently in place.
For others, it's sometimes possible to unplug the aerial lead until the TV has passed the unwanted signals. This is the case for a friend where they get a poor signal from a local infill relay at channels in the 40s, then the good signals from the main transmitter (Winter Hill in our case) in the high 50s and 60s. So the simple answer is to unplug the lead, set off the full scan, then plug it back in when the search reaches around 50.
Some devices have a function to rescan only the muxes it already knows about - that's useful as it means it can pick up channel changes, without going and finding stuff you don't want from other transmitters.

I had a "help" call from a friend not long ago. She has a very nice telly with sensitive tuner which picked up Moel y Parc in Wales as well as Winter Hill. Consequently she got a full set of duplicates - including what she termed "Taff TV".As I didn't have an attenuator to hand at the time (and also hasn't figured out that trick) I sat down and manually removed the duplicates - a right PITA and I wish the "powers that be" would come down off their lofty cloud and realise what a hassle it is when they force a retune on people. The TV doesn't have any selective scan feature, and the two transmitters have muxes on frequencies that are interleaved.
 
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The aerial you describe is a "Yagi" type, commonly known as a "contract" aerial (because it's cheap and cheerful and used by contractors).

There's no need to reduce the signal to zero - just to a level that the receiver can't detect. My guess is that 38dB attenuation should be sufficient to get it low but leave the wanted signal high enough. Some fiddling with the adjuster will probably be necessary but I'd start with it on maximum and reduce it if any wanted multiplexes are missing.
 
Well, thank you again for all the tips. I've now got a lot of options:- new aerial, attenuator, manual tuning, checking to see if the the programs are there in the 800s, and plugging/unplugging the aerial during tuning. Attenuators are cheap enough, so I've got a variable on order at the moment, and will try that first.
 
Attenuator hasn't arrived yet, but I went to my friend's house today to try out your other suggestions: First I checked channel 800 and, sure enough, there was BBC1 East, followed by ITV (presumably from Sandy Heath) on channel 801.

But my friend and his wife weren't too happy about having to go to channel 800, so I then tried a manual tune. The TV showed me a list of all channels it had tuned, so I selected only the ones I knew were from Sandy Heath (from the list in the link in an earlier post), and selected the "retune" option for each one. After I'd finished, BBC1 East was then on the TV's channel 1, and BBC1 London had been shunted to channel 800.

Thank you to everyone who posted with suggestions.
 
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surely once you have retuned to the correct transmitter SH with the attenuator in circuit to reject CP you can take the attenuator out of circuit thereby getting full signal from SH
 

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