I've just realised, it's worse than I thought. There's another HDD lying on the shelf, on which I had scrawled '
U/S; 600 bad sectors'. That is a Seagate ES.2, which is supposedly the Enterprise equivalent and contemporary of the 7200.11. That means the other 7200.11 is unaccounted for, I can only guess must have chucked it out, in which case it will have gone bad too. That would then make three out of three failures from Seagate, one supposedly Enterprise grade.
The Reg article is reassuring for me, as I have been gravitating towards HGST (Hitachi) of late, and these are what have replaced the failing Seagates. Most recent is a '1TB Ultrastar', purchased from Amazon circa £40 too replace last week's failure. Only disappointment was the manufacturing date printed on the label, which reads 2010 for a disk sold in 2015. But at £40 for a 1TB Enterprise (if you believe it) I still think it's a bargain, and probably wouldn't bother to claim under warranty even if I could.