Update on how things went.
In short - it was the other rear wheel bearing - as well ! They were both on their way and noisy, just the drivers side was noisier (further gone) ..but I changed the passenger side first. Hmmm.
In detail :
Noise from rear end started around 109.5K miles, Vibration first kicked in when doing 60mph plus. Over about 400 miles this came down to 20mph. After much listening and checking went for the passenger side rear wheel bearing. Once off this felt tighter to turn than the new unit - so I thought that was it. Wrong - noise almost as bad. Then came on this forum for much needed advice.
Got a quality replacement rear hub / bearing assembly (eventually - now disenchanted with ebay spare parts suppliers - another story) and fitted that on the drivers side. Noise gone - back to normal - relief. The one taken off was very tight compared to the new one - well on its way to full seize up.
On reflection: Its probably really hard to tell which side bearing is failing because the entire rear transmission and suspension is mounted on a sub frame. So the path for the vibration into the body structure is different from a lot of other vehicles - it just sounds a lot different. The advice on the side of the spare part box 'change both sides' is good - if you can afford it. Chances are if one side is on its way, the other will follow sometime soon.
Generally, as a DIY mechanic with 30 year experience well impressed with the engineering on this car: Great design and very doable ... if you have the right tools ;-).