Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
Your low definition BBC channels are sending that question to you. It wants you to do a full Auto scan to update your Channel List, and then it can store those new channel numbers in its list.
While you can do that to see if the new HD channels get stored, IF you aren't getting a strong enough signal on the HD channels, it may not store them. You may end up with the same Nag screen you have to keep dealing with on a daily basis.
The only reason a TV set would show a NO SIGNAL display on the HD channels is that the signal strength is low, or the new BBC HD channels are just coming online and they aren't transmitting it 24 hours a day, but only for maybe 4 hours in the morning or afternoon. That's because they are "breaking in" the new transmitter equipment.
So...you're kind of between a rock and a hard place so to speak. Do you do a full autoscan of your present channels so the TV stores them? What if they are only transmitting part time instead of full time?
And what if your aerial is strong enough to receive the Low Def channels, but you need an RF amplifier to boost the HD channel frequency up enough to not only LOCK the channels into that Program Guide but to give you enough signal power to actually SHOW the picture and sound?
Those are the questions we have to deal with.
RF amp needed?? Or just a new FULL Autoscan of available channels?
You can do the Full autoscan simply by selecting the TV's menu, and selecting the Autoprogramme or the AutoScan setting in that menu.
That menu also should have a MANUAL scan feature, and that usually has a SIGNAL STRENGTH meter in it. If you have 905 listed in the manual list, you can select that channel and see what your meter strength shows.
Now, I can only give you the basics because I don't have your TV's model number to work with.
But this is definitely where I would check for the Auto scan and the Manual programming settings --
The newer I manuals that Sony provides just aren't as "in depth" as the TV sets that had the Fully detailed PDF manuals. This came from Sony's EX series of TV sets, and was essentially the very first ones with the built in I-Manual.
Someone wanted to stop paying Adobe for PDF licensing, and this was their solution. Pay someone else for the I-manual design.
Alas, the information quality suffered because of that decision.
Anyways, try autoscanning the channels first. See if that gets you a good working BBC HD station or not.
Next, if you see it stored, select that channel, then go to your Manual channel settings and find the Signal strength bar. If it's over 50% power level, you should be seeing the picture and sound. If not, check back later to see if they may only be operating for 4 hours right now.
Lastly, you could compare your reception in your general area for the BBC HD channels...a neighbor, or a local TV store, or even a bar would show HD the same way you should be receiving it at your home.
Everybody has to go to market some time during the week. I do mine on Fridays and Saturdays. Take full advantage of what you have available to you. Plan a strategy, and work with me to find a solution to your dilemma, ok?
I honestly don't have a "one size fits all" solution when it comes to Aerial reception and your specific location. It's very dynamic and variable.