The musty smell that appeared when selecting heat after the car had warmed up must be a leaky matrix I surmised, so a new one was purchased (about Ã,£45) and I set about the task.
Removing the fascia was a time consuming job, once all the screws and bits of trim had been removed, not forgetting the wiper mechanism and pollen filter and its housing Manoeuvring the panel out was tricky but a thin piece of steel sheet enabled the passenger side to be pulled through without damage to door post or fascia.

[attachimg=1]

Once out I could see the top of the heater matrix but I hadn't the heart to remove the cross member and pull the whole heat and vent assembly back just to allow the two hose stubs to pass clear of the bulkhead.  I thought it should be possible to split the double hose at the flange and re assemble the new one in place. I cut the old one to save time after blowing the coolant out of the matrix.

[attachimg=2]

Once removed at least I had confirmation of the diagnosis, the matrix had a small amount of pink deposit at the top, it  smelled the same as the aroma which had been infiltrating the car. 

Here is the old matrix, very little liquid had escaped as there was only the pink deposit, no sign of any at all in the bottom of the plastic housing.
[attachimg=8]

The next problem was that the new heater matrix didn't fit, it has a metal rim around the top which wouldn't enter into plastic housing, I had to cut and nibble off the rim.
[attachimg=9]

[attachimg=5]

I was able to re assemble the double hose section to the top of the heater in place, using a bit of washing up liquid to ease the o ring entries and taking care to keep them clean so they wouldn't leak. A long screw driver could pass through the foam from the engine side do up the under side screw, (it wasn't too difficult to engage the driver in the screw with an assistant under the bonnet and a dentist mirror torch to see the under side of the flange.


The new one is in and leak tight, I went for test drive the get it hot before going to the trouble of putting the fascia back, oh and just to be legal I reconnected the instruments so I had a speedo.  Note that as well as not fitting properly, the screw holes didn't line up either.
[attachimg=4]

Another thing I learned once I had pulled pollen filter's housing out was that I had been fitting the filter holder the wrong way round, as the next picture shows, the two plastic pins, top and bottom should go in first, they then engage in 'tram lines' in the housing which guide the far end of the filter into position.
[attachimg=7]

By the end of the Sunday it was all back together and working well, but it was a lot of work and I don't know if I saved time by not following the book on pulling the HV unit back or not.


Best regards

Ian