What is the correct way to remove the number plate bulb holders?

It's in the owners manual if you want pictures... Do It Yourself >> Lightbulbs section (P.234 ish, depending on which manual you have). There is a link on the stickies at the top of the Octavia forum if your car doesn't have one. 

 

Lift the boot lid. Place a screwdriver / trim removal tool into left hand side of light fitting (forcing it to move right) - this releases the sprung metal tab and the light fitting can then be pulled downwards and removed. Installation is reverse, but should be able to do this by hand.

Thanks