lately i have noticed that my air conditioning when the car is stationary (in traffic or parked) is not blowing cold air as it used to. But when you are driving it works.

had the system topped up with gas and that hasnt done anything.

air compressor is noisy but works.

Air Con fan is on all the time it seems when air con is on

Radiator thermo fan comes on and off intermittently when air con is on.

I am thinking of getting a second hand air compressor to see if that works, but first wonder whether anyone else has had this problem and what they did OR has anyone worked on this problem.

The car is a V6 TE MANUAL

If the compressor is noisy it will be due to the internal bearing on the way out or the system is overgassed and the extra gas pressure when hot creates undue load/noise....although the compressor should switch off when high or low gas pressures are detected unless someone has bypassed the pressure switch.
My experience with 2nd hand comps has not been good - both times I've tried that they failed within 12 months and probably due to moisture creating corrosion inside the comp as the oil is hygroscopic...also when they pull them off wreck most places done seal the connections etc and even a grain of sand can wreck them.
I'd check that the condensor in front of the radiator is hot when you say it doesn't cool in traffic....it should be very hot. If it isn't it will mean the comp is not pumping the gas.
There are some good aircon diagnosis sites on the web so it's worth a search. Good Luck.