I'm going away for six weeks so I won't be throwing any parts at it in the form of the sensors just yet. That said I'm not sure it actually is the sensors.
I did get a replacement loom and installed the night before last. This of course was a battery out job. Once I had it all connected up I then ran a reset via MES. This failed on a reverse fault. I then took the car for a fairly long drive (about 20 miles) and it behaved perfectly, up and down shifted exactly as it should - no dropping out of gear, missing gears etc. But more importantly, I noticed a couple of other things. The engine ran quieter and stronger than it has been of late (I keep getting a rattling that sounds like detonation and the exhaust note has been off, and the power a little disappointing). And the petrol gauge that had previously had the light on and the showing a range of about 35 miles or so had suddenly jumped to a quarter tank with a range of well over a hundred miles.
Then I parked the car and turned the engine off. When I restarted it, all the symptoms had come back. It wouldn't shift properly, the engine was rattly, lower on power etc, and the petrol gauge and range indicator had dropped. I drove it around some more before parking it up and attempting another reset of the selespeed system, which amazing ran all the way to completion. But when started it up it still wasn't right.
This sequence of events happened once before (last week), where I pulled the battery, cleaned the various contacts for the selespeed unit and reset it whereupon it ran perfectly, until I turned it off.
I'm beginning to suspect that there may be an issue with a bad earth or something, given that the selespeed problem seems to be linked to these other issues, and under circumstances it be eliminated (on the first drive after a lengthy battery disconnect and selespeed reset).
I'm going to investigate it tonight and then this thread will be going into hiatus until I get back from overseas.