When the unit is held normally, that is with the antenna vertical, the
airport comms from the tower were really poor.
Lots of static and noise.

But, much better when held horizontally.
a. Any idea why ?

There are 10001 resons, but airfield communications ARE vertically
polarised. They tend to use discone antennas for VHF and the "biconical
monopole" for UHF. However, they are designed to broadcast to aircraft,
which do not sit so close to the ground and so far away. There is likely to
be an array of different antenna at the control tower and/or transmitter
site, all of which could slightly affect that which you are receiving.

The "rubber duck" antenna is NOT a 1/4-wave antenna at all. It is in fact a
3/4-wave antenna all coilled up, which gives a radiation pattern a little
close to the fictitious "isotropic" antenna. People playing with rubber duck
antennas on handheld gear rarely hold the things vertical, so the 3/4-wave
gives a more uniform radiation in all directions - especially slightly up
and down: more like a cylinder with a hole down the middle, than a donut.
Another point is that the impedance of the 3/4-wave is more constant when
your hands and head are close to the thing, but that is irrelevant for your
application.

Buildings tend to use metal structures, fittings, and wiring, all of which
affect the passage of radio signals through them, especially at VHF. I have
succesfully use existing mains conduit in the ceiling of a rented appartment
as an antenna, by mutual coupling of feed and resonant "current breaks".

Reflections from other buildings can also interract with the signal you
receive. You may think that you predominantly receive the main direct path,
but you would be surprised how often you are receiving reflections from
other sources. Most GSM and all later generation of cellular phone base
stations use polarisation diversity. That is one array of antennas
45-degrees to the left, and another array leaning to the right. All this is
necessary because reflections and movements change polarisation.

Q1: Are you sure that the antenna is vertically polarised?
To be vertically polarised then the end bit should be higher than the bottom
bit, and pointing at the sky. (Suoopid suggestion, I know, but you never
know :-)

Q2: Is the signal very strong? If weak then it could be arriving from any
source. If you building is shielding your then you may be relying upon
reflections from the bus park, aluminized billboard, power pylon, or the
neighbours TV antenna, for reception.

If you really want to receive these stations then use a simple antenna
outside, in a controlled environment, without any unknown influences. Such
an antenna need not be very complicated, in fact it could be built in a few
minutes using a bit of wire and a broom handle.

This link even does all the simple antenna calculations for you, for any
frequency you want.
http://web.telia.com/~u85920178/antennas/vpole0.htm

If you want to receive a specific band with decent sensitivity (weak signas)
then this link will give you some idea.
http://web.telia.com/~u85920178/antennas/6dbvhf0.htm

If you think that you have a weak signal, then see why at:
http://web.telia.com/~u85920178/data/pathlos.htm