...as working on the Rover.
But I just replaced the heater core on my wifes Volvo, what a PIA but I am glad that it is done.
Dealer wanted $1000, indie shop wanted $750, I did it in 2 hours and it cost me $145 for the heater and the coolant.

 

I remember one winter when I was in High School, my Oldsmobile had no heat. They told me the whole dash had to come out to change the Heater Core.

I froze that winter.

 

Yeah with this you just need to remove the carpet on both pass and drivers side, then remove a cover on each side, 5 screws total and then the heater comes right out.
Her car has the dual climate control just like the Disco so I am thinking the Disco heater is the same way, right in the middle of the dash behind/below the stereo.

On most cars the heater is buried deep inside the dash and the whole thing needs to come apart.
At my old job when I went into this shop they were replacing the heater on a Escalade, the whole dash was sitting on the floor, did NOT look fun.