First of all, thanks to Rich for pointing this out - I had no idea this part even existed!
After having a friend suffer with MAF failure on his ZT, and the new item going out of spec again a couple of years later, we deduced it was down to water spray ingress after a road test in heavy rain resulted in a fine layer of water droplets on the air intake and MAF pipework when checked afterwards.
It turns out MG Rover knew about this, and issued a Technical Bulletin with a fix - to fit a Water Shedder plate near the intake that would deflect water spray, and lessen the amount ingested.
The best part is, the fix is incredibly cheap!
The part in question is the Water Shedder Plate PHL000010
Edit: We now have these in stock, after a few PMs requesting the item:
Shedder Plate - 75/ZT Diesel
The bulletin is below:
Tech Bulletin - Water Shedder
Hope this helps some members - if there's enough interest, as usual I'll try and get a better price from XPart.
Normal retail on the part is £1.26 (plus postage).