Part of my touch screen isnt working

[Dj6]

It's along the screen we're the volume down button is ... I have not dropped it and its been in a case ever since I got the phone... I have been to O2 and they think I am bonkers...

Is there any way I can test the screen further then the normal phone screen test...? 

[Uliwooly]

@Dj6

Yes you can:

Settings > About phone > Diagnostics > Test

Oreo

Settings > Support > Tests

[dquilon]

What I don't like about the Touchscreen test that Uliwooly suggested is that the thickness of the touch is too big, so it actually mask small unresponsive touch sections.... it happened to my Xperia Z3 Compact and was a reason for me to lose a screen repair...

So, the better way to test is to go to the Engineering Test screen... by going to the Phone app > Dial > #*#*SERVICE*#*# 

that is: #*#*7378423*#*#

There you can scroll, to various Service Test that are more accurate than the ones available in Settings section.