A retailer is responsible for providing a repair or replacement on any item which fails to function due to an inherent fault (i.e. something which was actually wrong with the product at the time of purchase, such as the use of poor quality components or poor manufacturing standards).
Up until 6 months after purchase the law assumes that any failure of an item will have come about due to the presence of an inherent fault (unless there are clear signs to the contrary, such as the tyre tracks of the vehicle which ran over it), so the retailer must repair or replace the item.
Once 6 months have passed the retailer must still repair or replace any item which fails due to the presence of an inherent fault but the purchaser is then obliged to prove that it was, indeed, such an inherent fault that led to the problem.
So you would have to be able to prove that your pen failed due to an inherent fault in order to force WHS to repair or replace it.
I've never had a problem getting a defective Parker pen repaired (or replaced) by the manufacturer, free of charge, irrespective of its age. Give them a call on 0203 027 6892.