External LG monitor blurry after upgrade to new Mac Air, Sonoma 14

I've just upgraded to a new MacBook Air from my 2021 MacBook Pro.

The fonts on my external monitor, an LG HDR 4K, are now all blurry. Everything was crisp and sharp on my previous MacBook.

Both Macs running Sonoma 14.7.1. Settings on the laptops and monitor exactly the same. Monitor connected via an Apple usb-c adaptor with an HDMI cable.

I've seen some suggestions to unplug the monitor from the power socket, use Better Display, etc, etc, I've goodled and tried every suggestion and setting I can find, but nothing works.

This is driving me nuts - brand new expensive laptop, perfectly good monitor, all working fine before, but now I have this blurry low quality monitor thanks to the upgrade.

Any suggestions as to how to fix this are very welcome!