You need to be clear about the difference between a guarantee and your statutory rights.
A guarantee is a 'gift' from the manufacturer or retailer. As such, the company giving the guarantee is free to determine the terms of the guarantee (e.g. 'it runs out after 14 days').
Your statutory rights are totally separate and exist irrespective of whether any guarantee exists or not. Further, nothing written into a guarantee (or otherwise expressed to you) can void those rights.
With limited exceptions (relating mainly to expendable items, such as batteries) you have the right to a repair or exchange (but not to a full refund) of any item that falls below acceptable quality WITHIN 6 YEARS of purchase if the problem is due to an 'inherent fault' (such as poor manufacturing standards).
WITHIN THE FIRST 6 MONTHS after purchase there is an automatic legal assumption that any fault occurring will be due to an inherent fault (unless, of course, there is indisputable evidence to the contrary).
THEREAFTER its necessary to show (based upon the balance of probabilities) that the problem has occurred due to an inherent fault. (If you'd gone past the 6-month cut-off, a written statement from your plumber regarding the manufacturing fault should suffice).
It is the RETAILER (and NOT the manufacturer) who is responsible for ensuring that your legal rights are complied with. So the retailer is obliged to repair or replace the shower cabin. Given that it seems that an effective repair has proved impossible, it would seem reasonable to expect a replacement cabin.
Further, the retailer must seek to ensure that the purchaser is not left out of pocket, or otherwise unduly inconvenienced, through the provision of a repair or replacement. (e.g. if a retailer has to send a TV set away for a repair which will take several weeks, he should provide a set on loan for the interim period). In your case providing a replacement shower cabin would presumably mean that you'd have to pay the plumber to take the old one out and put the new one in. THE RETAILER SHOULD REIMBURSE YOU FOR THOSE COSTS.
As your original post suggests, and Sandy's link acknowledges, the credit card lender is equally liable with the retailer but I hope that this post helps you to understand the rights you have anyway. (Don't be fobbed off with statements about 14-day periods!).