Just go here
http://support.seagate.com/customer/warranty_validation.jsp
or start here if you aren't sure of the model etc.
http://www.seagate.com/support/service/index.html
They will usually get you a new drive within a couple of weeks - well worth it.
If it says seagate on the external box then they should honor it (it could be a maxtor in which case they will also replace it)
What brand drive was on the inside of the thing?
If it is not a seagate then inform seagate that PC world are selling counterfeit product, I'd also write to the UK computer magazines and tell them what happened, they love stories like this and quite often get something to happen...