Bench seat plastic back fixings?

[SimonSpain]

Hi everyone and HNY 2013!!!

I was of course 'fiddling' today as it's the 1st Jan. and my other half is working, so I thought I'd look at a stupid error I made when I tried to fold down the rear bench seat when it was pushed back. I was looking at how it would fold with the rear parcel shelf removed, and didn't realise that you have to remove the fixings for said shelf or the seat will get caught on them when you fold it down...which it did!
It folded up again with a bit of a crunching plastic sound, but all was ok, just a a small scuff. But today I saw that it had pulled the lower left corner of the plastic back away, so you could see the white plastic screw or knurled fixing that keeps that corner in place. I lined it up and knocked it back in with the palm of my hand, and then heard a clack and the end of the white plastic 'screw' came off. It has held fine, whether by the tension or enough of the 'screw' left, but being a clumsy idiot AND also a perfectionist, I feel the need to change out that broken 'screw' when I can.
Anyone have any ideas on how easy or difficult that would be...and to get that small white fixing 'screw'????