The rear heater Matrix leaked a few months ago. I tried to get another and bypassed it, which suited me fine. Eventually found a replacement on ebay in excellent condition and fitted it last weekend;
As I couldn't find any info on fitting, here's my experience.
Removed bench seat to access panel trim behind drivers seat.
Removed trim and unplugged switches and connections.
carefully remove the 3 x nuts from underneath (by the matrix hose connections) holding the matrix housing in place (mine were not seized as It was under-sealed well).
Removed the fan blower from inside.
Popped off the clips holding the top of the housing to the bottom unit (saves removing the coolant
connection). Did this with a long screw driver.
Remove matrix and replace.
Reverse the process. The clips are a fiddle but by loosening the nuts initially gives more movement.
I have some Photos of the process but can't figure out how to post them

I am going to sell my Bongo as I don't use it enough (I converted it 3 years ago and done under1000 miles). Almost sold before but it is such a great drive I kept it. Has to go now though.