Question

How do i replace the 850 watt electric ultrastore heater in a truma hot water boiler

Answer

The heater is attached to the water tank with four nuts, two for the heater and two for the thermostat. I believe these are 5mm and 7,5mm metric nuts so small socket spanners are required. Before removing the nuts make sure you disconnect the supply wire and there is normally a push fit connector in the under seat locker.
You can peel back the cardboard casing to access the nuts. Remove the small nuts trying to ensure that you do not lose them in the gap between the casing and the tank. Remove the heater.
Fit the new heater ensuring that the rubber seal, supplied with the heater, is in place.
The push fit connector is not easily re-usable and you may wish to use a standard 20amp junction box to re-connect the supply wire.