I had a second hand set put on my ZT-T last year - the originals were completely rotten. The rest of the car is very clean underneath, too!
I fitted the second hand set as I'd acquired them some time ago in return for a favour - glad I held onto them now.
For anyone needing replacements, I have them here:
Rover 75 / MG ZT Rear Upper Suspension Arms
They're direct from XPart, and are genuine items.