Tried the disk in a caddy and still no joy, also tried it in the freezer as per hellsbrink's suggestion. I've even tried tapping it with a hammer.
When the drive is powered up via the caddy, it makes a beeping noise - not clunks or graunches, it does this for about a minute, then stops, so it's obviously getting power OK.
Presumably the beeping is some sort of initialisation processing where it's internally detected that there's a problem.
Any other ideas?