Underside protection fasteners (help please)

[M1 KRX]

This is difficult to describe but on my driver side two of the plastic fasteners that hold the protection cover on to the underside of the car are are missing. Does anyone know what they are, clips or simple plastic nuts?
if they are nuts then what's the tread size.

Thanks in advance.

 

[Chrishazle]

They could be "plastic rivets" - with a central core that pushes in to expand the serrated split outer parts and lock the fastener in place, and can be pulled out to release the fastener.

 

[M1 KRX]

Thanks I Guess I need to get a closer look to see what's missing then.