Stephen wrote:

Now that I know from the "one metre drill bits at Aldi" thread that
these bits are flexible, is it possible that the bit deflected inside
the wall?

Yes. I used to drill deep through-holes in steel. It was rare for the
drill to stay square to the bed for more than 8 inches.It could be half
an inch out on a foot long hole. That is in steel, which is a pretty
consisitent structure thoughout. Concrete has all sorts of holes and
stones in it to deflect drills, so it would be unlikely to get a true,
straight, level hole through such a length.

How can I prevent this in the future? I drilled from one side straight
through. Should I have carefully marked each side and drilled from
each side, hopefully meeting halfway?

For drilling though brick I have used a 16mm bit for 15mm pipe in the
past. Do I need to use something bigger than 16mm in concrete to allow
for it not being straight?

Yes. 20mm would be suitable for 15mm pipe, maybe even 22mm.
Alan.
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