I have a A3 1.8 Auto AGN 1997 which the previous owner advised the air con is not working.

This is correct and its blowing warm air.

Could someone help me understand what the Diag Codes are saying.

I have run the On Board Diag and get the following

Channel 52
Air Con switch is off:
Get following segments lit.
a1 Slippage or blockage, A/C Refrigerant High Pressure Switch, 120x off - Seems to be the cause ?
b1 Engine Speed (RPM) less than 200 - 500
c1 Engine Speed (RPM) greater than 6000 - rather strange when car is idling at less than 1000 rpm
d1 Engine Speed (RPM) greater than 6000 - rather strange when car is idling at less than 1000 rpm
b2 low voltage - What's at LOW VOLTAGE ?
c2 Kick-down switch, compressor off for 12secs max.
g1 System function OK (g1+g2+g3 must illuminate simultaneously to indicate system is OK)

Turn air con switch on.
Get following segments lit.
a1 Slippage or blockage, A/C Refrigerant High Pressure Switch, 120x off - Seems to be the cause ?
b1 Engine Speed (RPM) less than 200 - 500
d1 Engine Speed (RPM) greater than 6000 - rather strange when car is idling at less than 1000 rpm
e1 System function OK
g1 System function OK (g1+g2+g3 must illuminate simultaneously to indicate system is OK)

Channel 53
Air Con switch is off:
Get following segments lit.
a1 Fan for interior temperature sensor
b1 Fresh air/recirculation flap closed (recirculation mode)
c1 Heater valve closed
d1 Bi-directional wiring harness
e1 A/C compressor ON - Rather starnge when we know its off
f1 coolant fan first speed ON
b2 Air flow flap open
c2 Air flow flap closed
b3 Central flap in "instrument panel outlet" position
c3 Central flap in "footwell outlet/defrost" position

Turn air con switch on.
Get following segments lit.
b1 Fresh air/recirculation flap closed (recirculation mode)
c1 Heater valve closed
b2 Air flow flap open
c2 Air flow flap closed
b3 Central flap in "instrument panel outlet" position
c3 Central flap in "footwell outlet/defrost" position

 

Also Decimal Point does not come on.

 

Garage confirmed compressor is faulty.

Looks like its going to be wind the windows down.