Dead right by g-rover. I've got a 1989 RR and i did my heater matrix last year. G-rover's spot on, take your time, mark the wires and allow a weekend. Two things i'll recommend tho cos i learned the hard way.
1: When you get to the point of moving the heater unit out, the thermostat wire (silver, quite stiff) that goes across to the aircon unit just pulls out of the aircon end. I cut mine in a panic and it took ages to find a new one.
2: Also invest in new coolant pipes that go in/out of the heater matrix (back of the engine bay, fit onto matrix at the bulkhead). I tried to re-use mine and ended up with a footwell full of coolant.
It might make the job easier if you take the top of the dash out while you're there, it'll make manouvering the heater unit out alot easier. I didn't and ended up scratching it with the heater control sticks whilst waggling the unit out.
Makes my job sound like a bloody disaster! Wasn't too bad tho, ended up with a few nice new parts installed.
Good luck taking the heater unit apart to, more screws in that than i've had hot dinners. Hope it goes ok.