setting margins and date format for pdf export

[Katie Eldridge]

Dear all,
I'm trying to customise the taxinvoice report.  I've managed to do 
everything I want, apart from:

The pdf (A4) seems to have a 10mm margin except for the bottom margin, 
at 20mm.  As I've added a footer, I'd like to make this margin smaller, 
but can't see anywhere to set it.

I'd also like to change the format for the invoice date and due date, 
but again can't see where this is set.

I'm using Gnucash 2.6.11 under Linux Mint 17.3

Many thanks in advance for any pointers.
Katie