I've started a new thread since this is becoming somewhat surreal :-(
I sent a WD Blue 2 TB drive back to WD under warranty, having gone through their web site and followed their instructions, including getting an RMA.
It seemed to be taking a long time so I found a customer service email address and chased it up. Credit to WD they have been answering emails pretty well by return.
Having finally got Royal Mail Tracking to work it said "The consignment was sent back to the sender - refused acceptance by the addressee."
In fact the SSD arrived through my letter box at lunch time and the return reason was given as "Unclaimed" so presumably WD in the Czech Republic failed to collect it from customs?
I advised WD CS and told them I had sent a second SSD, also with a RMA and they replied "I assume that the same will occur with the second shipment, therefore try to get back the second parcel as well." They went on to say "Once you have the drives you need to replace let us know so we can create a new RMA and provide you with all documentation along with a pre-paid shipping label that will cover the shipping cost."
So it seems I can hope for a good result but they are so sanguine about what happened, almost as if they expect it!
I will hang on now for the second drive to come back...