This means that when l open the filler cap air escapes and l can only get 24 ltrs of fuel in at a time even if the gauge is showing empty.

The air rushing out is normal.
Check the Tanks capacity in the manual, standard 206 may have a 24L tank..

If its not holding as much fuel as it should then the tank may have a dent in the underside or teh gauge may be showing empty when you may have 10 or more liters in there, try running it out of fuel . dont forget to take a fuel can full of fuel with you..

Is it sucking air in when you remove the filler cap? Perhaps the breather in the filler cap is blocked.

The fuel runs under pressure that is what the air escaping noise is its perfectly normal. What 206 is it petrol or diesel and what engine cc? They normally hold about 50 litres and if its only taking 24 litres thats really worrying.

Sounds like the breather tube is blocked, check the shape of the tank underneath the car . If a vacuum is present when fuel is used this has distorted the tank and reduced its capacity. This happened to me with a car once.