Apologies for seeing this late in the day.
I had the same problem last year...sounded like a jet engine starting up at 60mph onwards. After a few weeks the noise would start at 40 mph.
Replaced a bearing in the Haldex under warranty and no problems since.
Sorry for all the grief you are getting !

Forgive my ignorance, is replacing a bearing in the Haldex different to replacing the rear diff pinion bearing?

Sorry mate - don't know. I have had two haldex repairs, one was an oil pump and the other the whining bearing. as they were done under warranty the dealer didn't give me any paperwork I can refer to to answer your question. I would suspect they are one and the same.

Low level repair is to replace the rear differential pinion bearing. If that doesn't cure the noise the next stage repair is a replacement Haldex unit.