Thanks again Tsquez, that’s really helpful. I think we missed a few updates along the way but your improvements make perfect sense!
For the record, here’s what I’ve added to the child theme stylesheet to make our site (the one I’m using as a test case) look nice again with 5.5.1.7:
.row { max-width: 62.5rem; background: #FFF; }
#header { padding: 0; }
Also, the main sidebar widgets disappeared, I think because at some stage the ID of “Main sidebar” was changed from “sidebar-1” to “main-sidebar” – so I changed the reference to it in sidebar.php of the child theme and it showed up again.
Finally I also went into Appearance > Customize and:
* In Header Section > Site title & tagline, unchecked the “Display Header Text” box
* In Colors > Top-Bar Colors, set the main colour, because it was set to #333 and over-riding our child theme style for menu buttons
* Also set some sidebar link colours that had changed with the upgrade
* NB These are just issues I noticed but presumably I should go through all the settings in the Customizer and make sure they match the site branding etc
I think that means we can stick with 5.5.1.7 and future versions now. Thanks for the steer on where to look.