I'm trying to hang a sliding glass door. It runs on a heavy metal bar
which is attached to the wall at 4 points. The kit provides 4 screw-in
fastenings which have an M8 threaded section which sticks out of the
wall and the bar bolts to.

I've used 12 mm sized plastic plugs as per the instructions but my
drill turned out to be slightly bent so my holes are sloppy and not
too accurate either. They are drilled either into a concrete lintel or
into concrete blocks.

How do I make this good? I'm thinking along the lines of Rawlbolts
which might be firmer, but how do I ensure that they are correctly
centred?