800 are usually duplicate channels of those assigned to the 100 range.
The weather is playing havoc with signals at the moment so it may be that this is affecting your usual transmitter but not those duplicates.
It's not possible to reasign channels on TalkTalk boxes, these are fixed by Freeview.
You could try a retune, but be aware that if it's weather related you could lose more rather than restore those missing ones.
Try taking the aerial out and retuning first though as this clears the box memory, then retune with the aerial back in the box.
Hope this helps.
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