It's a fiddly job that requires calm, tea and plenty of breaks.
They tend to wep a bit so the hot water comes out in small amounts and pools in the bottom of the heater box. When you set the temp to warm it opens the flap to direct air through the heater box and the air that comes out is full of moisture due to the leak and therefore fogs up your windows.
There's a uide on here on how to do it. If you're local to Basingstoke I'll happily give you hand as I've done about 5 of these now.
nick