Okay - an update by those waiting with baited breath

First off, at Nick's suggestion, I tried re-ripping one of the Sibelius disks which I know I have already successfully ripped, and the behaviour was indeed identical - ie. the draw automatically opened to eject the disc, though there is no error message to explain why (though I already knew!!).
I then tried unchecking 'Skip duplicate albums' and restarting Control:PC (as suggested by Reg and Rick). I first checked that the changed setting had been saved (as there is no 'apply' or 'save' button) which it had.
I then tried ripping one of the Sibelius disks which had failed and ejected this morning, and it immediately worked.
I have not yet tried any of the other options suggested.
Interestingly, with 'SDAs' unchecked, I did get a dialogue box, which correctly confirmed that the disc in place was Symphonies 2 & 3, and I did (I think) get to 'accept and transfer'.
I note there is now a tick box in settings 'Automatically approve albums with complete metadata' and this is ticked.
I wonder if this option didn't exist in the previous build, and the adding of that option, which was automatically ticked, is what made me feel like I had lost the control which I previously had - with the clearly flaky 'de-duplication' functionality only compounding that view.
Musings...
Cheers
Neil