Just looking back over my posts, and realised I'd never written my own answer to my original question.
The vibration from a failing front flexdisk, was stressing the exhaust-to-gearbox mount.
Once the vibration became serious enough to track down and replace the flexdisk, I realised what had been going on.

As a matter of interest, the exhaust bracket initially failed at least 3 years- 75000 miles before the flexdisk problem became noticeable.

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W202-C220D ( UK ) 310K miles