Thanks Armand for your two posts. Unfortunately The Gnarbox is listed but not available at Amazon in the UK, and seems only available at a very high price from one company. I also read that Gnarbox as a company has had or is having problems, so I am a bit doubtful about it.
I am aware of the Western Digital unit, but while it is listed on the WD UK website, no one seems to stock it!
Best wishes,
Jonathan