My issue turned out to be a hardware fault.  I went through Dell support and they ran through the normal diagnostics with me and then dispatched an engineer under the premium support option.  48 hours later the engineer replaced the entire screen and inbuilt camera.  Fired it up and worked first time. I was convinced it was a driver issue but I guess the hardware can also fail.  
A few weeks ago the webcam just stopped responding and all my teams call switched to audio only. I have run support assist and the camera does not show up as a piece of available hardware to troubleshoot. It is enabled in the BIOS and has no errors currently showing in device manager. It is listed as Intel(R) ADL AVStream Camera. I have tried all available versions of this driver, all were unsuccessful. Intel 2D Imaging/MCU/Visual Sensing Controller Driver for Camera | Driver Details | Dell UK I have today performed a factory reset and run all available updates and still no joy. Is this a known issue on windows 11?  
XPS 13 - Windows 11 Webcam fails to start or be recognised