Can anyone tell me the best way to remove these? Swapping number plates over at the weekend and one of the screws that go into the threaded rivet had rusted solid and would come out no matter what I tried. Ended up drilling the thing out and buying a new rivet from stealers to replace the damaged one but then I've been thinking about removing the other 3 and having the plate just flat to the bodywork.

Or indeed how to fit the new rivet?

Or better yet, buy anti-seize from an auto parts store, coat the bolts and your problems are over.

I'd still have the issue of removing the existing rivets, the have what can only be described as a 5mm washer that makes the plate sit proud of the bodywork

First of all, are you referring to a screw or a rivet? Big difference. Screw is threaded, rivet is not.
If it is a screw, spray something like WD-40, or penetrating oil on the screw and let it sit for a couple of hours.
Spray it behind the washer.

Both are threaded, the rivet has a thread to take the bolt, the rivet is pretty much like a rawl plug that's in the boot lid to secure the plate using a bolt. I tried wd40 but the rivet was just spinning hence why I had to drill out the bolt and the rivet came away with it a little bit. I've looked at it more closely today and the new rivet will not stay put in the hole like it should so the bodywork itself has been slightly damaged when I tried to get the bolt out, I've been and bought some steel araldite, hopefully that'll keep it in place, failing that I'll just get a rubber bung to fill the exsisting hole and buy some 6mm foam tape to secure the plate

This is a photo of the replacement rivet....




You can see the part I mentioned earlier that's like a washer, and on the second picture you can see where the thread starts about 6-7mm down

Since you now have a new rivet from the dealer, fit that rather than removing the other 3.
Can you get at the rivet from the inside to help secure it in place?

I assume you have the chrome mounting plate that affixes to the rivets? Mine has a sticker with the VIN on it.

I removed my plates last weekend also because I noticed one of the bolts was rusting. I replaced the screws with plastic ones.

Managed it this morning, the hole in the bodywork had enlarged so the rivet wouldn't sit properly in the hole!! Good old araldite! All hidden by by rear plate, so all is good ��