For a Td5 you will struggle to find a part number... as non EGR equipped Td5's are only supplied to South Africa and a couple of other countries that get RoW spec Defenders... AFAIK they are not listed on Microcat.
300Tdi's are slightly different, as when launced it didn't have the EGR fitted, and came with a blanking plate. And they are listed on Microcat.
(don't think 300's are the same size as Td5 - but not sure)
You could go on a quest to try and find the part number....
But the easier option would be Allisport, Devon4x4, Bell Services (or Ebay).... all of them do EGR replacement bits and pieces.
All well as blanking off the feed from the exhuaust manifold, you wil get a improvement going the whole hog and removing the EGR throttle balde on the inlet manifold - again the usual suspects do a replacement section.
Ian