Hudson Reed TSVT005 Topaz Traditional Bathroom Shower Triple Concealed Shower Valve with Diverter, Chrome & White

I was persuaded into getting one of these by the Bath Store. However, after getting in three different plumbers to look at why the bath water was luke warm, I discover that the maximum temperature is in the late 40s and therefore the bath cools down by the time it has run. It works fine with the shower.