I was washing my car earlier and noticed the bottom screw of the front liner was loose where it connects to the sill. The screw won't tighten up at all, should there be a clip for it to screw into? 

Did you try backing it off and tightening it again?

I did, I took it out then put it back in to see if it was an alignment issue. 

Think you’ll need to remove the liner and see what’s stopping the screw from locking-up. 

I've had a look it just goes into a rectangular shaped hole in the sill, that's why I'm wondering if something should be there. 

Photo showing exact location would allow parts catalogue look-up.

 

I think I've found it, the screw that isn't tightening is the one with the no. 7 next to it. 

https://nemigaparts.com/cat_spares/etka/skoda/sup/768/821100/

 

It looks like it should be screwing into an expanding nut (the bit that has the number 8 next to it) 

 

 

So the expanding plastic thingy is missing?

8 hours ago, numskull said:

So the expanding plastic thingy is missing?

It appears to be. I'm wondering if it went walkies during an inspection. 

It would appear that u r missing #8 from the diagram.

Ordered the part from ebay and the job is jobbed