Ruth - you can get the bigger ones to go through if you wizz a 4mm drill bit through the hole.
The hole in the plastic is actually 4mm, but once it has all been painted it makes it slightly smaller and you have problems getting the bigger bolts through.
You can buy the proper size screen screw kits from Pro-Bolt for about 14 quid -
http://www.tastynuts.com/ - make sure they send you the right ones though as they're specific Ducati kits (3mm bolts) and not the standard ones you've got (4mm).
antonye
Threaded Mode