They are closest to 'Push Rivet 8mm (33)' I think. I'd need to check the size though.

I think I have the part number wrong though as Google comes up with an upper control arm bolt which I also have on the same invoice. I'll verify this evening and also measure the rivets I have :)

And they did. +++ I had a look at the head of one that wasn't missing and found a similar-looking one in my stock.
It's like a mix of #40 and#63 so either would probably work. #33 seems too long.
It worked fine for the wheel arch liner. :cool:

I've stickied this thread now as a useful reference resource thanks everyone +++

Worth noting, as eBay listings are rather "ephemeral" to say the least that the items referenced above are listed as "Audi trim clips".
This is the sellers shop front: http://stores.ebay.co.uk/Car-Workshop-consumables and as they have sold nearly half a million items I guess should be around a while.

The correct part number for the rivets is 4D0807300 :)

Those look identical to what I used - no idea where I got them though.
Picture from ECP:

Interesting, today I decided to look at the undertray having sorted out the wheel arch liner the other day.

It looks like the main fixings are secure, nothing moving in the centre, though I can't get underneath to check in detail.
It also appears that the rear of the bumper has been fixed (screws?) across the centre to the front of the undertray. So that is secure if not "correct", anyone know what that "should" be?

But the connection at the bottom of the wheel arches (two each side) has just been lashed with cable ties and is pretty loose! :tuttut:

Groping around I could feel some sort of retaining clip, but not as large as the ones I ordered as specified above, so I cut the cable ties and had a closer look.
Extracting one of the retainers it was clear it had been broken at some time in the past, so could not do its job as intended - the section that the pin locates in and locks by turning was missing!
But it does confirm that we have identified the correct parts. +++

So tomorrow I'll have a go at fitting it all together properly with the correct fasteners. :cool:
See photo which shows the new pin and clip with the old (part-missing) clip alongside.

The front of my undertray is free floating, having nothing holding it in place, other than being on top of the lower bumper panel. Going by the size of the holes, I think it should be secured with the same type (plastic) as the ones in your photo.

Well I'll have whatever I don't use with me when we tour Ireland in June......

I've been wondering about this today. It looks like the undertray should have another five of those plastic 1/4 turn fasteners at the front of it. The tray has slots for the receptacles to sit in, and the bumper has matching holes. So in total it has 9 of the plastic ones, and 4 metal ones.