What size thread are the captive nuts for holding the rear number plate?

 

Also, how deep are they. Short of pulling off the inside boot trim and measuring, I can't remember how far they go in.

 

I've had a search but can't find anything.

If they've not been unscrewed for a while. you'll most probably find they're rusted in, solid.  I used that double sided tape to attach a new rear plate, it also looks far better with no unsightly screw heads/caps showing, and avoids the risk of cracking the perspex plate when you drill through it.

Mine hasn't got any screws in at the minute, it hasn't had for about 4 years.