Well I've eventually found your post (under 'Paying by Credit Card', rather than on one of your threads with 'Shower Cabin' in the title!).
My advice is to take photographs (or a video) of the examination of the shower cabin taking place, ensuring that the problem areas are clearly visible. (If you're using video, getting the engineer to clearly state that's there's a problem would be advisable). I know that you've already submitted photographs of the problem but getting evidence that the company's engineer agrees with you could further add to your case.
Also try to see if a reasonably independent person (such as a neighbour or even someone from your local CAB) could be present.
Many companies will try to duck out of their legal responsibilities up until the very last moment but when they realise that you're not going to back down they'll suddenly back down (because it's simply not worth their time and money to argue any longer). That may well turn out to be the case here.
Good luck!