Hi @Mosey, let me see if I can help out here.
Has this just started happening or has this been occurring for a long time? Also, have you been able to carry out a quick test of the touchscreen to see if the screen responds as intended? To do this, find and tap Settings > Support > Xperia Tests > Touchscreen.
Another thing that you can try is to reboot your phone in safe mode to see if you experience the same behaviour there. All apps downloaded from Play Store is temporarily deactivated while in safe mode, that way you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
If the back button behaves the same way in safe mode, I would suggest that you try to repair the software on the phone using Xperia Companion and a computer. Keep in mind though that all user data will be erased by the repair, so it is advised to do a backup before to avoid data loss.
Try these measures and let me know how it goes!
