A quick update for those who have not lost the will to live.
To cut a very long story short, I discovered the flashing screens were down to the v-format button and now the only problem that remains is the dropping and/or breaking up of (mostly HD) channels i.e. pretty much full circle.
I've tried the humax with two different aerials and with two different freeview TVs - one SD one HD. Both TVs worked perfectly off both aerials but the humax problems persisted in CF or SF mode. I did notice a change in reported signal strength on the humax in the two locations and also when the pass through RF to the TV was removed. (In one location the strength went up after removal, in the other location, the strength went down!) I then remembered an ancient Tesco special offer Teknika HD box i had stored away and connected that; it worked perfectly in both locations.
I'm not a technical person, but i feel sure that there is a problem with the tuning hardware on the humax - it seems to need a particularly strong signal compared to the other devices. Whether this is a 'feature' or just old age, i guess it's the end of the road.
On the bright side, although the Teknika does not have a meaningful favourites feature, i can delete channels from the EPG. And it has some other useful features so i'll probably use it as my 'front end' when i move imminently to a 4k TV.
Many thanks to all who contributed.