If the only channels affected are BBC 1 & 2 then it may b a freeview signal issue.
Have you tried a full retune? Do you have a standard freeview TV you can test bypassing the youview box ?
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We have the same issue. You can get the one HD and all the other channels, but not the normal BBC channels. This happens now and then. I normally switch to the TV's freeview and its fine, so it has something to do with the youview! I do find talktalk services very poor.