For some reason my 2002 fabia has started to relock the doors after opening with the remote, just as though it had been pressed by mistake without the door being opened within 1 min. Also the warning alarm for sidelights being left on and drivers door opened  when key is removed has also stopped working.Could this be a faulty switch on the doors? Does anyone know where the door switch is?

Will likely be the micro switch on the door latch that is at fault. I'm sure I have seen a guide on how to replace it on this site somewhere but for the life of me cannot find it?

Definitely a failed microswitch in the driver's door lock unit. Very common issue. You can't buy the switches separately but if you are good on the tools you can fit a similar switch from the likes of ebay like I did. It's still working over a year later.

 

http://www.briskoda.net/forums/topic/330840-door-microswitch/

 

If that's not an option you will need a whole new lock mechanism.