Because the item from Rimmers, as supplied by Rover, is the pump and all the other bits and pieces. The MKON is just the pump meaning you have to extract the old pump from the total gubbins and then fit the pump to the rest of the gubbins.
"Don't think of them as problems, think of them as opportunities." "OK, I think I've hit an insurmountable opportunity!"
An alternative is an ITP element from a 306 HDi. Our black one has one fitted as it matches all the specs of the orignal pump. You have to do some dissassembly and swapping bits about, but total cost is about 50 quid.