Having found no exact answer on the web, I've done some dissasembly of a a spare.[br]
To the left of the sony logo on the Z5 and Z5C, you can see an oval window. On the left inside this, is the proximity sensor - a tiny dot about 0.3mm dia. next to this is the ambient light sensor - about 1.5mm dia.[br]
The sensor needs to be close to the front glass. I found that over time, the top right corner of the screen had become unglued from the frame. sticking it back fixed the problem - although other ppl have used a bit of packing between the main camera and the body.[br]
the sensor is part of the flexable pcb that carries the buttons and vibration motor.[br]
You can test this by going to phone then dial ``*#*#7378423#*#*`` to bring up the diagnostics menue.[br]