This is the final post from me I guess. The card still works sometimes, but it is not at all reliable. I get exactly the same behaviour in my friends Foxsat HDR, so it's not broken. I've tried all available firmware versions, including custom firmware. To make it 'most likely' to work, I have the following procedure, but even then it's at best 1 in 5.
Turn device on
Tune to an encrypted Tivusat channel
Turn device off
Plug in a USB key (at the front) - I discovered this because I have left it plugged in after upgrading the firmware
Put the CAM in (but the card in upside down - i.e. image up)
Turn on (I have only had it work when switching it on at the front, not with the remote, but I don't know if that is relevant)
Device will (sometimes) start and say that the TiVuSat card is in upside down
Turn card around and it works.
It you go to the menu system and see 'tivusat' under the Common Interface menu item it is working. If it says 'Empty Slot' and then 'Initialising..' you need to try again.
I really doubt much of the above procedure is relevant and basically think there is a race condition (bug) in the software during initialisation which Humax don't care about since no one uses CAMs in this device. I am putting mine back on ebay and have bought a iCord Mini instead. No iPlayer and EPG, but I have a playstation so I can live without that.
It anyone else finds a real solution, or Humax update the software again to fix it then please let me know.