Have now swapped the drives over, and the new one works perfectly. I don't even have to send the old one back! Although I should now buy a new IDE cable - the new drive actually supports ATA-100 which needs an 80-wire cable to run at that speed.
Full marks to Plextor for quickly arranging the replacement. I discovered the failure on Monday, reported it on Wednesday, they shipped the drive yesterday and got the replacment today. This is the sort of service companies should provide.