its starting to sound like blockage - I wonder if pulling the thermostat and lower hose to shove a garden hose in and flush from both sides maybe even the heater inlets as well.... would that be a crazy idea?
- I did that on toyota years ago with no issues
-edit- I made sure the little whole in the thermostat was top center for air bleed.
It can be frustrating thats for sure but theres alot of very knowledgable people here. Stick with it - I love my 98 disco Hope you get is sorted

 

Heh, I'll be happy to take it off your hands, it's got a lot of new parts on it. I'll make you a very reasonable offer...

Seriously. Don't let this thing kick your ***. Old Rovers are not for those who cannot persevere.

It's either making too much heat, or not removing enough. I vote for not removing enough. So far, you claim to have eliminated pretty much everything but the radiator as a culprit. Unless the water jacket is all plugged up with goo or crud, but lets hope thats not it, cause if it is you are screwed.

Anyhoo, you can borrow a temp gun from autozone (at least I can) and look for cold spots on your radiator. It's FREE!!

Now don't go ordering a new radiator just yet, slow down and trouble shoot it out. It costs less.

 

Put the old water pump back in...

 

Like others have said, bleed hole in t-stat at 12 o'clock and if you got zero bubbles and burps something is funky. How confident are you in the work done by the radiator shop? Have another shop flow test it? And yeah t-stat quality sucks, I had two out of the box bad when chasing down my last cooling system issue.