Faulty Shower Tray

Sunnyside1_2

Hi,
I'd like some advice on my consumer rights in relation to a shower tray purchased in January 2012. The shower tray was not installed until about 1 month ago and has now since developed a crack across the waste part of bottom of the tray. The shower has not been used and is currently not even tiled for use. I have contacted the supplier (internet supplier) who requested photos of the fault and have since notified me that they cannot replace the unit free of charge as the fault was not reported within 28 days of purchase. The trouble is that evidence of the fault was not present at the time of delivery and the fault was not revealed until after installation. The shower tray was installed exactly in line with the manufacturers recommendations. It doesn't seem right that they are not liable for failures that occur more than 28 days after purchase, basically offering no warranty or guarantee at all.
Help!