@stephenuk Had you tried to set the port for the HDMI Parameters within the config.txt? From the Raspberry Pi Documentation
Because the Raspberry Pi 4B has two HDMI ports, some HDMI commands can be applied to either port. You can use the syntax <command>:<port>, where port is 0 or 1, to specify which port the setting should apply to. If no port is specified, the default is 0. If you specify a port number on a command that does not require a port number, the port is ignored. Further details on the syntax and alternatives mechanisms can be found in the HDMI sub-section of the conditionals section of the documentation.
