wrote:
Thanks for all the advice so far

Here is a prelimanary plan of the new extension (left of existing
house), dimensions (red lines) are approximate:
http://img73.imageshack.us/img73/1793/205gh.jpg

There is a 4" soil drain which will go under the rear wall of the
extension. Do I run the footings to either side of the pipe leaving
approx 6" gap and then the brickies will place a lintel to bridge the
pipe?

Yes. That is one way. Another, after you have checked your levels, is to
run the pipe OVER the footings and construct a brick and lintel arch
over it.

In general bricks should go from perhaps a couple of courses below
ground level to a couple or three above it, before you insert the DPC,
which is usually at or about finished floor level.

The higher your soil pipes can exit the steeper the run they will have
outside. Conversely the shallower they will have inside..so its a fine
point as to where they should go,

BUT try always to get to 1:60 drop. You *can* get away with less, but
thats a good target to aim for.

You should gravel it up as well