Remember the 9kw heater is a 2 Man unit. 
It’s designed to be able to provide 2 users, very hot water. 
One user is always going to get very hot water from it no matter the weather. (If it’s set up correctly) 
I have to run mine on minimum setting in the winter, and then in summer I can adjust it higher, 
Or I can adjust it higher for a Pvc clean etc. 
I use the grippa hot water hose to, but I fancy trying my normal Hi vis hose. I find the red hose quite hard to manage.