That may weaken your case a little depending on how long ago!
When retailers play this game they hope you will become fed up and they can avoid a straight returns procedure.
Assuming you have all the paperwork, your only avenue now is to deliver a written ultimatum to them with the request to settle now or face a civil action via the Small Claims Court.
Keep all paper work and whatever emails you have.
If it comes to a court action you must be prepared for more expense and hope they settle first. Small claims courts are not a certain cure but with a little bit of luck may be something PCNextDay would like to avoid.
[It is all down to Customer Service and the firm's attitude.
I pushed Novatech for a replacement CDROM for one of their older laptops. By the time it arrived I had learned that it was major surgery involved, and they accepted this return at full value because "I had not seen the drive when ordered". Now that WAS service and I still happily buy from them.]