As a footnote to my original post and for anyone who might be faced with a similar situation, the following will get Serif WebPlus X5 satisfactorily working under Windows 11 (well, it did for me)…
Right-click the shortcut you use run Serif (or maybe the .exe file itself, although I haven't tried this).
Choose Properties.
Go to the Compatibility tab.
Click the button Change high DPI settings.
Tick the box Override high DPI scaling behaviour.
In the dropdown below it, choose System (Enhanced).
Click Apply then OK.
Close and reopen the app.
Good luck if you're reading this after experiencing the same problems I had. Long live WebPlus X5!