A3 door closed sensor problem

My A3 thinks its driver's door is open. This only seems to happen in warm weather - it was a problem last summer but during the winter has been fine and in the evening it's fine when the temperature is cooler. When you close the driver's door the interior light stays on and the DIS display on the dash says the door is open. Repeated opening and closing doesn't clear the problem. Also, the central locking button inside doesn't work and the alarm doesn't arm when locking the doors from outside (although the door itself does lock).

I have taken to climbing in and out from the passenger side so as not to open the driver's door!

Any ideas? I can't see a switch anywhere round the door so I assume it's part of the door lock. Do i need a complete new door lock or is it a dodgy sensor that can be cleaned or adjusted? Happy to tackle it myself.