Thanks for that Kyle, that helped me get it working.
I found a good explanation of how Java maps logical fonts to physical fonts using font configuration files here. This explained that the fontconfig.Ubuntu.bfc file was taking precedence over the fontconfig.properties file. When you delete the Ubuntu version of the file the system then defaults to using fontconfig.properties.
I noticed that your version of fontconfig.properties has an xlfd name for dialog.plain.latin-1 ofdialog.plain.latin-1=-b&h-arial-medium-r-normal-sans-*-%d-*-*-p-*-iso8859-1
I found a tool called xfontsel which lets you create these names - this shows that Arial comes under monotype. As far as I can see you also have to have an entry in the Font File Names section for each flavour of Arial i.e. normal, bold, italic and bold italic. I’ve attached an updated fontconfig.properties file which includes these changes.
fontconfig.properties (18.0 KB)