Ive had to replace the heater core in a Disco 1 '96 300tdi twice in 3 years. The first change was a $30 2nd hand part. It lasted 3 years so not so bad.
Firstly, you don't have to pull everything out like the manuals say which I did the first time. I did learn a lot about disco dashboards but some things were not meant to be learnt.

Remove the low range knob, hand brake handle and centre console. Remove the cover piece below the steering wheel (undo the hinge bolts). Remove the metal bracket that runs along the lower edge of dash (where those hinges screwed in). and remove the angle bracket which runs over to the transmission tunnel. This is about 6 bolts. The floor heater outlet may be in the way too. Remove that. You can now unscrew (one screw) the bracket which supports the two heater pipes. And you can loosen the two clamps which hold the pipes into the heater core. The bottom of the dash is plastic and can be forced towards the seat, out of the way. (I put a rope on it and tied it towards the seat base). You need to do a bit of levering to separate the pipes out of the heater core so you can pull the heater core out. Be brave, it will come! Basically, pull the heater core and pipes out a bit then lever them apart.

I found it all incredibly difficult to get back together the first time and impossible the second time so I cut the aluminium heater pipes leaving enough pipe to put a rubber heater hose on. I could then re-attach the pipe ends into the heater core out of the car, used silicon etc to get a good seal. I cut the pipes inside the car a bit shorter so there would be room for the two short lengths of heater hose. I pushed the rubber heater hoses onto the pipes inside the car first, easily slid in the core and with some difficulty pulled the rubber hoses over the pipe stub ends attached to the core. And I remembered to put the 4 pipe clamps on first! It leaked a bit at first where the aluminium pipes went into the hoses. I've had to tighten the pipe clamps twice, but it now works. And the next changeover will be easy!

I have heard of someone (on this forum?) replacing the aluminium pipes with rubber right through the firewall. Next time maybe.

I would love to have pics of all of this but I was too busy upside down under the dash and swearing a lot.

Good luck with yours.
Lachlan