A couple of flat head screwdrivers eased under the opposite edges of the central button should get this moving. It should reveal a plastic pin with a flat top. Once these are removed completely the outer button should ease out.
I find a squirt of ArmourAll helps ease these clips out without too much fuss.
Thanks for the tips guys that answers that question.
If this is a push in plug part do you reckon a screw can be used in its place to hold the mud flap on?
If I can use the supplied screw that will come with it I will use it, I just meant I wasn't sure if there is any depth of material for a screw to "thread" itself into and bite - behind that lining wall?