I eventually got back to my faulty machine yesterday and made some progress. Previously I had thought that the unit was searching for channels but nor accepting any. Having overcome the triggering problem with my scope, I can now see that instructions are indeed being sent to the tuners via the I2C bus both during an auto tune and a manual tune, but a "no channels detected" message is received. While I was recording the waveforms, I repeatedly set an autotune as this gave me lots of time to set the scope up and record many transactions. I decided that the tuners were probably all right.
I was just about to pack up for the day when I noticed that some tens of channels had appeared,, all labeled "unknown" and appearing in the 8oo series. All were completely scrambled and unwatchable. The hidden menu indicated that they were from Channel 24 (Winter Hill local) and 39 (Winter Hill SDN).
This morning, to my surprise, I successfully tuned all Winter Hill Muxes manually with no problem, everything in the right place and solid as a rock.
Suspecting bad connections or temperature problems, I spent the morning with a hair dryer and ice cubes in plastic bags, all to no avail. I even tried a method used by an old friend who used to fault find valve TVs with a wooden lollipop stick, loudly singing "Strangers in the Night" "Knock, Knock, Tap, Tap". He claimed it always found such faults!
Could all this have been caused by some corrupted data base which has been fixied by the current channel changes?
I will leave the unit on soak for a few days and see what happens.
PS. All the above was with the original tuners refitted.