dnsmasq.conf.add lost the YazFi lines, you mean?
At the moment its expected that changing Guest Networks in the GUI will "break" YazFi until it is manually run but the conf file should remain intact
Yes, dnsmasq.conf.add had lost all the YazFi lines (the files are attached to my previous post in a zip - just remove the additional .txt extension). I'd not made any changes in the GUI until after the guest network stopped working. Removing and re-enabling the 2.4GHz guest fixed it.