Anyone had any experience with the door contact/switch on these cars?

My mates MKIII (2002-2008) Ibiza has developed a fault where by the car doesn't seem to recognise the drivers door being opened. The interior light does not come on when the door is opened and the automatic relocking thing happens even though the door was opened. Functions fine with the passenger door so I'm assuming it isn't a fuse thing, more likely to be some switch/sensor around the drivers door. Central locking and electric window is fine.

I had a look around the door frame but couldn't see any obvious faults or where the switch is - is it built into the door lock?