It's an easy job unless you have aircon because the bolts that hold the heater to the footwell are hidden behind the aircon. AND it is glued on at the factory so getting it out can require a crowbar. I changed the element in mine which means you have to take the whole shaboodle apart. The idiot that designed it ensured that the plate that holds the radiator element in will, sooner or later, wear it's way through the alloy radiator. I wrapped the plate and other bits in rubber before reassembly. What you read in the Haynes manual is how to change the heater motor only. The Haynes manual is rubbish on the heater, but so is the LR Manual - 'remove bolts' when you have no idea how to get to them. Many Defenders here in South Africa have the heater bypassed (mine was for 2 years) as new heater elements are expensive and second-hand ones are no better than the one you take out, plus it is not a nice job to change the heater box. I was lucky in finding someone who made a trialler so sold me the heater for about a fiver.