Thanks to everyone for all the useful advice so far. To respond to
various helpful comments in the onbe place:

- I am not drilling near a door or window.

- My test holes are around 20 cm apart, both vertically and
horizontally

- I am using a masonry drill bit that should certainly be up to the
job (sorry, don't have spec to hand at the moment)

- I'm drilling into a brieze block from my garage. On the garage
side, I am about 3-4 feet above ground level. Since my kitchen is
slightly elevated, this should bring me out about 15cm above the
kitchen floor level.

In the same region, there is already a pipe coming thorugh from the
kitchen radiator, so I guess I'm probably looking at a more powerful
drill. Failing that, I might just bite the bullet and get someone in
that knows what they're doing )

Cheers

Mark