Screwfix Triton Delphi Thermostatic Mixer Shower

We bought a chrome Triton mixer shower last August and had it professionally installed in October. It has only been cleaned with non abrasive cleaner. At the end of August we noticed the chromium riser was pitted and the chrome had started to come off. This is supposed to have a 5 year guarantee. Contacted Triton who refuse to exchange it as it 'is longer than 90 days'. Rubbish shower and rubbish service. Avoid at all costs.