hi if its the tank heater then its a 3kw so u cant get anything bigger,if its the v2 heater they hav two one is set at about 65 the other is adjustable up to 110 and the only u can get higher heat is by having the 110 v2 heater as this heats the water after the pump the 3kw tank heaters are just like kettle elments and are capable of boiling the water but are set to 65 because they heat the water before the pump and any higher heat will ruin the pump
on your machine dos the light come on when u turn the heater on? if this is the case id say the thermo is working and so is the cut of switch and most prob the elment is bad, if the light dosnt come on id checj the plug fuse and if it is okay id say its the thermo thats bad or safty cut out in tank maybe bad or stuck open
imo your better of just with the v2 heater as these will allso heat the tank up if u open the flow control , i hav both but never use the tank heater as it takes half hour to heat water up and the v2 heats tank up lot faster