Well it came back to life!
Last year I took the drive out of its protective anti-static bag, to see if I could do something with it. I noticed that inexplicably, some thermal conductive paste had become smeared over part of the logic board. I removed the paste but some of it was still lodged in vias, I plugged it into the modified external enclosure/caddy and tried reading the disk. It spun up and stayed spun, I was then able to read off all the files I really wanted to keep and then attempt to copy the entire disk, I got every last bit of data off that drive and onto a much new 3Tb unit.
So what happened? Holy intervention? Aliens? Anti-Gremlins?
I think the initial failure was entirely to do with the logic board, which had perhaps overheated, or through lots of heating and cooling cycles had fractured a solder joint between one of the components and the PCB. This is at least partly to do with the ROHS certified lead free solder that the logic board is held together with, its the same thing that causes Xboxes to die early deaths with that annyoing red ring.
I believe that when I cleaned off the conductive paste some of it became lodged in whatever cracks had appeared, re-bridging the connections and allowing charge to flow around the board and through all the components again.
Presumably, if this is the problem then the fix will only be temporary, but could be fixed permanently by reflowing the IC's, however I've now put the drive to a more dignified rest along with many others in a storage cupboard, in a stolen green tesco crate. Even if i reflowed it it would not be reliable enough for any serious use and 500Gb really isn't much these days anyway!