Well after the failure (again) of the Fusion Drive in my iMac I took it to the Genius Bar of my local Apple Store. They confirmed my diagnosis that the Fusion Drive had failed.
I had a few options.
I could replace the SATA HDD myself with another SATA HDD, which I did consider. I could replace the broken SATA HDD with a new SDD drive.
I could get someone else to do that.
However upon consideration, taking a 2014 iMac apart isn’t a simple job and would require replacing the adhesive for the glass LCD panel. So wasn’t sure I wanted to go down the road. Also the cost of a 2TB or 3TB SDD was quite expensive, though buying a 3TB HDD myself would be cheaper than what Apple was proposing to charge.
I did consider before taking it to Apple to get someone else to do this, but they would charge £84 first just to diagnose the problem, so would then charge (like Apple) for labour on top, as well as the replacement HDD cost.
What the Apple Store was proposing wasn’t excessive and so decided to go down that road.
So now they have my iMac for a week, well hopefully less time than that.