I would have thought that they were at fault. I guess you have to prove that either the materials or the workmanship was defective. Im not sure how they can argue that this happens from time-to-time, and if so why do they offer a guarantee?
My first step would be to contact them in writing stating that you believe that this is covered by their guarantee and that if they do not fix them, you will contact FENSA to ask for their advice. Keep a record of the letter and when you sent it.
There is a site called How To Complain (type that into google to find it) where you can get help with what to say. Hope this helps!