Well, Stu, I know that what I'm going to tell you won't be helpful, but as a matter of interest I'm using an iP7 with iOS 11.2.6 and everything is working fine.
That being said, and knowing that you have already checked your settings and read the troubleshooting steps provided in my previous link about mics, it's difficult for us to give you an appropriate answer remotely. We are just users like yourself.
I would suggest that you contact your most convenient Apple Store or Authorized Service Provider and have your device inspected. They will diagnose the issue and give you all the availble options.
Apple - Find Locations - Location Selector
Kind Regards
Giulio