aquatron 4 low pressure on output?

aquatron 4 low pressure on output,when the temperature dial is turned to required temperature, the output flow reduces dramatically often causing to drop into low pressure mode which results in cold water output.If you turn dial to cold, pressure very strong. Temperature returns if switched off then on again, but still remains low flow. Mains pressure well above 3 bar.Can I carry out any tests to clarify which replacement part I can order.Sower working fine till 3 weeks ago