Channel 108 should have a picture all the time - even if it's only "preview" programming. As regards, the Welsh channel - presumably you put your postcode in when installing the Humax,
The problem may be that the dish has moved a smidgin - but it seems odd that it should just be the BBC HD channel that's affected. You might try downloading it in non-Freesat mode - the frequency etc are here -
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By the way, for full functionality from the HDR it should have two feeds from the dish - though it can work quite well with one, if you follow the instructions carefully.