I live on the south coast and have had this problem as well. I got the humax, moved house and got a new aerial connected at roughly the same time so can’t do any before and after comparisons. I must be getting the signal from Whitehawk.
Anyway, the HD channels come and go for me. Usually they are there but sometimes it says that “there is nothing being broadcast or it is encrypted”. If one go then they all go. The meter at the bottom of the channel info screen indicates no reception.
Most annoying.