Use the smallest drill bit you can get hold of to start with 3mm is idea. This will go straight through and not get deflected so much. Then you can move up to a 5mm bit, then 7mm. Its a bit drawn out but it will guarentee you a good fix rather than take chances.
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Hi depending on what you want to fix you could use a thunderbolt or rapier fix. These fixings will drive straight into light weight blocks quite easily with a cordless driver. If you haven't used them try one in solid masonry for which you drill a pilot hole. No rawl plugs required!