Hi, have a look at this link, I get fixings from them, if you scroll down to bottom of page it tells you what size screw, rawlplug etc go together, hope it helps you, Ray
The idiots' guide is often stamped on the plastic sprue that holds the plastic plugs together - both the diamter of the hole to drill for the plug size, and the size range of the screw options possible - as nos. 6, 8, 10, or 12.