There is a way and it's been posted on a forum somewhere. It was from the inside, you can see the matrix from the fuse box area (with a torch), first the fusebox is removed to get access and the pipes have to be cut shorter but the matrix just slots-in IIRC, it took the poster only an hour to do although it's a fiddly job to do it that way.
I believe thats how the LR main dealers did it when there was problems with a iffy batch of RRc heaters were fitted back in 1990s.