Thanks so much everyone. I've just ordered a water thermometer to check the temp.
In the meantime - can I ask if you know what the 1-5 numbers are for on the thermostat - are they temperatures or something else?
I have a UV Gold 170L unvented cylinder with 2 x Diamond H thermostats which are both set at 4 out of 5.
My water is incredibly hot - can I just alter the 2 x thermostats to 3 out of 5 to lower the temp of the water or is it not that simple?
Many thanks