The tests that I did yesterday afternoon (post 48) were carried out because I was sure that it was the source, not the TV, that was doing the aspect ratio switching as so far I can find no reference to aspect ratio switching via HD HDMI apart from 2.35:1 in HDMI 2. Every reference says that HD HDMI is 16X9. So all I had to do was switch between a 16X9 and a 4X3 channel, (using the Humax in" Auto/wide" and its O/P set to 1080i) and check the TVs "Screen Format" menu didn't change, to prove that, despite what was being displayed, the TV thought it was 16X9.
I failed in this task, watching the Drama Channel switching between Terry and June in 4X3 (did we used to watch this stuff?) and the 16X9 comms, the screen format menu changed from "normal" (4x3) to "full" (16X9) at the start and end of the commercial break.
I'm happy to listen to anyones theories as to how this happens, no doubt metadata and hand shaking are involved. Possibly the Humax outputs a pillar box image plus metadata that changes the format menu but nothing else, who knows? I'm sure the tests can be easily repeated using other equipment, let me know your results, remember you must check your screen format menu (or whatever it is called) to be sure it is the TV that is changing and not the source as visually it is probably impossible to tell, also check that the image is 1080 not 576.
Ignore the bit about the old 20S3020 and the Talktalk box, it's a very old set, I included it out of interest but it is a red herring as far as current sets are concerned.