Here's what Z wrote on the subject  with some comments from me

Z>>Replacing the heater matrix on a LHD car is almost trivial. You need
Z>>to remove the right panel on the central console and look under it.
The side panel is hold by one nut hidden in a hole in the carpet in rearmost part of the panel (close to the seat), and by a screw in the airduct (in the panel) for the foot compartment

You'll also have to remove the hard and soft panel under the dash, as well as some of the plastic air ducts under the dash.

Z>>It will probably be self evident what needs to be done.
Z>>The matrix is hooked up into two metal pipes that bring the coolant
Z>>through the bulkhead. They 'plug' into the matrix end, and seal by
Z>>means of an O-ring each. They are retained on the matrix by a metal
Z>>retainer that fastens with a screw (or two, I forget...). 
It's just one screw.

Z>>When you unscrew it, have some kind of pot or can ready as coolant will come
Z>>out.

>>The matrix itself is mounted into a 'slide-in' support within the
>>climate control assembly, it's only held by two clips ...
and one or two screws

>>...that you have to bend outwards, and then the whole thing slides out.
It is just possible to slide it without damaging it too much, but only after the cable and its clip are loosend and pushed aside.

The price of the heater was around DM 150,- in Germany