Can anyone explain to me the difference between a centrifugal and an axial
fan. In various catalogues, it appears one type is better at pushing the
air
along ducting but all the fans seem to be just "propellers" spinning.

A axial type is the "propellor" like system. Good at producing a high flow,
but poor at compression. i.e. they can't sustain a pressure difference.

A centrifugal compressor looks nothing like a propellor. It consists of a
spinning plate with strakes emerging from the centre to the outside. Air is
introduced near the centre point of the plate and is spun outwards and
collected at the edges. This creates considerable pressure difference, but
moderate flow.

The advantage of a centrifugal fan in a ducted system is that the ducting is
resistant to air flow and needs a considerable pressure difference to be
effective. A single axial fan just can't produce that sort of difference and
effectively stalls, moving no air at all.

For a straight through the wall system, a suitably rated axial will usually
be fine, unless the wind effects on the house mean it has to overcome a
pressure difference.

Christian.