My theory was proved correct last week when the heater matrix let go. Seems it had been leaking for a while, but then suddenly gave way and deposited hot water all over my drive.
So off I trotted to the local Land Rover spares place to get a new heater matrix. (I should really get shares in Brookwells the amount I spend with them!)
1: Removing the heater box
Firstly remove the two water hoses that feed the heater.
The heater box is held in with 4 bolts - 2 at the top, and 2 at the bottom. Its a 2 man job to undo the bolts, as the bolts are in the passenger footwell - unless you have 10ft arms of course! You will also have to remove the 2 cable adjusters on the heater box.
The heater box will then wiggle out. It took us about 5 minutes to get the heater box - its a tight fit, but it will come away eventually.
2: Removing the heater matrix
There are 2 types of heater matrix - the early type (mine) with turned down pipes, or the later type which is slightly larger with horizontal pipes.
Technically the heater is not a serviceable item, but replacement matrix's are available. This means you have to drill out the rivets on the heater box and bend the metal back to get the old one out, as shown in the photos here:
As you can see from the small pile on the floor - a lot of the foam around the heater matrix had disintegrated over time. This is allows cold air to bypass the heater and results in the typical luke warm air flow that Land Rover heaters are known for!
3: Fitting the new heater matrix
When fitting the new heater matrix, it is wise to fit new foam around the matrix itself so that it seals properly and doesn't let cold air passed.
The best way I found of doing this is to cut a strip from an old foam camping mat. I wrapped this around the edges of the matrix and then slotted it into the heater box as shown in the photos below.
It was a nice cozy fit, so no cold air can bypass the heater matrix now. The camping mat foam won't break down over time either.
All that remains to be done is use some self tapping screws seal the top on the heater box again. (pic below)
4: Refitting the heater box
Refitting the heater box to the Land Rover is pretty much the reverse of removal - lots of jiggling it around until it slots into position.
I bought a new heater box to bulkhead seal which made it more difficult to fit and I had to really force the box into position to line up the bolt holes - again, a 2 person job to get them all lined up.
All that is left to be done is reconnect the heater adjustment cables and reconnect the water pipes. Add some more water to the cooling system as the engine is running to purge any air from the system.
I am pleased to report that not only is my heater much more efficient now, but the vehicles coolant temperature is much more stable now the water system is pressurising correctly.
Another good job done.