A few minutes of trying variations on a theme resulted in this handler which does exactly what I need ...
on makeNewPlistFile(folderAlias, filename, initialRecord) -- name of initialRecord is "xxx"
try
return ((folderAlias as text) & filename & ".plist") as alias -- the file already exists
end try
set newFilePath to (folderAlias as text) & filename
tell application "System Events"
set rootItem to make new property list item with properties {kind:record, value:initialRecord}
make new property list item at end of property list items of contents of rootItem with properties {kind:record, name:"yyy"}
make new property list file with properties {contents:rootItem, name:newFilePath}
return ((path of the result) as alias)
end tell
end makeNewPlistFile --------------------------------------------------------------------