Got xperia 5 compact bought from singapore which worked fine for a few months then screen blanked when making a call. Did software repair via sony companion and reinstalled software. No apparent difference however I find that when phone ringing if I push power button twice quickly and see home screen symbol at bottom of screen . Press it and swipe down top left and you can see a call tab with an 'end' command which if pressed will end the call if you wanted to! 
Hi @Craftycockney, I am very sorry to hear about this.
There is a sensor at the upper part of your display (see item 6 in the overview of the phone) intended detects if your ear is within proximity or not, and if it is then your display will go dark during calls. This is to avoid you accidentally ending calls with your cheek for example.
You can test the functionality of the sensor using the Diagnostics tool that is pre-installed in your phone. In order to do this, find and tap Settings > About phone > Diagnostics > Tests > Proximity sensor.
Do you have a display protector mounted on your device, or any cover which could cover the sensor? Try removing it in that case to see if the display. You can also try to clean the area around the sensor with a dry cloth and see if the phone behaves the same during calls.
If these measures do not manage to improve the behavior of the display during calls I would suggest that you contact your local support team in order to have your phone sent in to an authorized service center. If you have any questions regarding the service, such as routines, warranty and consumer laws, these type of questions are best answered by your local support team as well, as they vary between markets.
I hope this answered your question. If there is anything else that I can help with, feel free to ask! 
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