So our shower will sometimes give you a lovely consistent shower. Other times it will go hot and cold on you, for no obvious reason, ie no other stuff like dishwasher/washing machine running which would interfere with the water flow. Shower is on its own heater, 9.5kw. Sure its something a bit obvious that I am too stupid to see 
Please @PigletJohn or someone who is more diy minded than me (not hard). TIA.