Whenever I open the drivers door the interior light doesn't come on but will work if any of the other doors opens (including the boot).

Is this the same across the entire Fabia range?

Does the buzzer make a noise if you open the door with the headlights on? If not, it could be that the microswitch in your drivers door is bust.

Didnt know the Fabia had a buzzer for when you leave the lights on (as I drive with my sides on during the day).

Where would this micro switch be located?

The microswitch is in the door lock, have a look here:

VWvortex Forums: MKIV door locks explained - why you're having problems!!! :(

Also, check the multi plug connections for corrosion at the a pillar first. A search will bring loads up.