Well, I still have the problem but am getting closer to finding the cause!
I noticed after the new driver
install that all my settings had reverted to the default ones. It is this that caused me to believe that the problem was resolved.
The setting that is causing the problem is the speaker configuration (and I thought I'd tried this - see first post!). On any other setting than 2/2.1 (i.e. stereo) the problem persists. My preferred setting is 7.1 because that is my speaker setup but it also "fails" on 5.1 etc.
So we know it's a sound issue re 2.1 vs 7.1 but that doesn't explain the whole problem. Let me describe the scenario when testing above...
Watching recorded programme, an episode of Homeland on 4HD, with the speaker config set to 7.1 (or 5.1) and audio OK. Ad break starts and audio OK. Ad break ends and programme starts - no audio. Click programme back to the ads (audio returns) and then stop programme. Change speaker config to 2.1. Click "Resume" on WMC and ads play with audio, ads end and programme restarts and we have audio.
So far, pretty clear but this next bit is the confusing part (as described in an earlier post). Repeat the above sequence to be sure and recreate the problem by having the speaker config as 7.1 and the programme restarts after the ads with no audio. Now stop the programme without clicking back to the ads and the choose to "Resume" the programme and the audio returns even though the setting is still in 7.1 mode.
So, the cause, whatever it is, occurs when passing from advertisement break to a transmitted programme on an HD channel when not in 2.1 speaker config mode. I'm sure it does this at other times as explained in the first post but at least this condition is repeatable because I have the recorded programme to test with.