OK, for those interested, I have an answer... and the LNB / splitter / cables is innocent [sic]
From TVHeadend forums (only found via Google search) someone has had the same issue. The config file for the tuner is in error / incomplete.
Workaround is
Go into the /home/.hts/tvheadend/input/linuxdvb/adapters directory
There are your adapters listed - they are all xml files so 'human readable,. You can pick out the adapter name at the top of the open file. Three of my files were c1658 bytes; non configurable adapter was only 658 bytes. Using text editor I copied in the missing section starting ' "Satconf" : { ' then saved file
Restart TVHeadend (for me : 'sudo service tvheadend restart'
Check TVHeadend Web GUI and 'unconfigurable adapter' now has switch listed...
Edit setting and 'voila' - it works..