If your mower is a Champion R484 then this is your answer:
Spare parts are available for this mower from 01889 561500
The black plastic "knob" with a nut in the middle is part number 22399900/0 (pack of 4) the nuts are part number 12293200/0 (pack of 4). Alternatively do a search for a Lawnking R484 which is exactly the same mower under another name.
Hope this helps
Gavin Hughes
June 2007