Hi John. our S5915 is M6 x 60mm and you would start off with a smaller pilot holes with something like 4, 5 or 6mm, then go to 12mm for the clearance hole. It does depend on the type of substrate you are drilling into, for mortar you might want to go slightly smaller depending on the quality of the mortar but for standard facing brick 12mm should be ok. Blockwork can be tricky depending again on whether its an low density aerated block, Thermalite insulation blcok, medium duty aggregate block or a heavy duty 7N block. Best advice is, if not 100% sure, drill a test hole in a brick or block to confirm. These expansion bolts are not suitable for work requiring sign-off by Building Control or  Local Inspectors such as balustrades/ overhanging pelmets or attaching load bearing objects. I trust this helps?