The warranty replacement may well have been provided to PCWorld by the maker of the hard drive rather than Dell
This is one of the quirks of warranty replacements. Invariably the terms of the maker's replacement policy will stipulate that the original unit must be returned in exchange and if you wish to keep the original unit you must, instead, pay for the replacement
In actual fact Dell do have an extra which may be ordered with a new Dell computer which provides cover for exactly this situation but it is mainly the business community that uses it
If the drive has been returned through the usual channels you are probably quite safe but I know that may not provide much comfort
Why don't you write complaining to PCWorld Chief Exec emphasising your original stipulation that the old drive should be returned to you?