Just discovered that the automatic PDF invoice generation didn't work because the logo image was in the wrong format. :( I'm pretty sure I tested this when I set up the site. Hmmm... Thankfully the automatic HTML invoice was OK. :) My apologies to those who got an error message instead of the expected PDF invoice...
I have now used my alternate square logo design and saved it in a compatible format, so both PDF and HTML invoices should now work OK. Still not too happy about the layout, though. Something to fiddle with on a late night...? ;)