Fender Extender

[Frosties]

You are right Frosties - stop drilling is a proven method, but its only temporary.
We use that method on aircraft all the time but at some point the item has to be replaced as the crack finds another route from the stop drilled hole and continues to failure.  Granted the stresses on airframes are more severe, but the same effects are in force on the mudguard/fender.
In this case though I think clayt74 is right - probably overthinking this.....its just a couple of bits of plastic in the end 

Think you're right Grant - sod it, might just rough it up and use the silicone then  :thumbup