newly fitted triton thermostatic bar mixer shower. model number-40744-0001?

I had this new shower fitted triton bar mixer thermostatic type. It was fine for the first couple of weeks amazing infact. the last couple of days it has started hot enough then goes teppid the pressure seems to dance all over and no matter how high the temp is even on full it just stays teppid/cold. this is such a winnd up and is not a gud start to the days plz help anyone would be gretaful.
the details are.. sfxlucthbm,,,40744-0001